Move self to the explicit () parameter list (proposal #7016) (#7272)

Implements proposal #7016: `self` moves from the deduced implicit list
(`fn F[self: Self]()`) to the front of the explicit list. Its type may
be written explicitly (`fn F(self: Self)`) or omitted, in which case it
defaults to `Self` (`fn F(self)`, `fn F(ref self)`); `self` in the
implicit list is rejected.

Throughout checking, `self` is modeled as the first explicit parameter.
Because a method is just a function whose first parameter is `self`, it
can also be called as an ordinary function with the receiver passed
explicitly (`Type.M(obj, ...)`), not only as `obj.M(...)`. A new
`SemIR::CallArgParamPatterns` helper chooses the parameters matched
against the explicit arguments, excluding a leading `self` only when it
is supplied as a method-call receiver; arity checking, conversion, and
generic deduction use it. The resulting SemIR and lowering are
unchanged: `self` is still `call_param0`, and witnesses, thunks, and
vtables are unaffected.

An omitted `self` type is parsed as a `SelfBindingPattern` node with no
type expression; checking synthesizes the `Self` type so it behaves
exactly like `self: Self`. However, the exact spelling used must match
between a forward declaration and a definition, following #3763's rules
around declaration matching.

Generated functions, thunks, and C++ interop import/export build `self`
as the first explicit parameter, and the `self`-type override (e.g.
Derived->Base for a virtual override) applies to the explicit `self`.
Placement is validated by new diagnostics: `SelfInImplicitParamList`,
`SelfNotFirstParam`, and `SelfOutsideParamList`. The benchmark source
generator and the documentation adopt the `(self)` shorthand; the
prelude, the examples, and the test data are migrated in the following
commits.

Assisted-by: Claude Code with Claude Opus 4.7

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>

toolchain/check/testdata/function/call/fail_explicit_self_param.carbon

@@ -1,67 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/none.carbon
-// TODO: Add ranges and switch to "--dump-sem-ir-ranges=only".
-// EXTRA-ARGS: --dump-sem-ir-ranges=if-present
-//
-// AUTOUPDATE
-// TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/function/call/fail_explicit_self_param.carbon
-// TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/function/call/fail_explicit_self_param.carbon
-
-// CHECK:STDERR: fail_explicit_self_param.carbon:[[@LINE+4]]:6: error: `self` can only be declared in an implicit parameter list [SelfOutsideImplicitParamList]
-// CHECK:STDERR: fn F(self: ());
-// CHECK:STDERR:      ^~~~~~~~
-// CHECK:STDERR:
-fn F(self: ());
-
-fn Run() {
-  F(());
-}
-
-// CHECK:STDOUT: --- fail_explicit_self_param.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: constants {
-// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
-// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete]
-// CHECK:STDOUT:   %pattern_type: type = pattern_type %empty_tuple.type [concrete]
-// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
-// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
-// CHECK:STDOUT:   %Run.type: type = fn_type @Run [concrete]
-// CHECK:STDOUT:   %Run: %Run.type = struct_value () [concrete]
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
-// CHECK:STDOUT:     .F = %F.decl
-// CHECK:STDOUT:     .Run = %Run.decl
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
-// CHECK:STDOUT:     %self.param_patt: %pattern_type = value_param_pattern [concrete]
-// CHECK:STDOUT:     %self.patt: %pattern_type = at_binding_pattern self, %self.param_patt [concrete]
-// CHECK:STDOUT:   } {
-// CHECK:STDOUT:     %self.param: %empty_tuple.type = value_param call_param0
-// CHECK:STDOUT:     %.loc19_13.1: type = splice_block %.loc19_13.3 [concrete = constants.%empty_tuple.type] {
-// CHECK:STDOUT:       %.loc19_13.2: %empty_tuple.type = tuple_literal () [concrete = constants.%empty_tuple]
-// CHECK:STDOUT:       %.loc19_13.3: type = converted %.loc19_13.2, constants.%empty_tuple.type [concrete = constants.%empty_tuple.type]
-// CHECK:STDOUT:     }
-// CHECK:STDOUT:     %self: %empty_tuple.type = value_binding self, %self.param
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %Run.decl: %Run.type = fn_decl @Run [concrete = constants.%Run] {} {}
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: fn @F(%self.param: %empty_tuple.type);
-// CHECK:STDOUT:
-// CHECK:STDOUT: fn @Run() {
-// CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %F.ref: %F.type = name_ref F, file.%F.decl [concrete = constants.%F]
-// CHECK:STDOUT:   %.loc22_6.1: %empty_tuple.type = tuple_literal () [concrete = constants.%empty_tuple]
-// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete = constants.%empty_tuple]
-// CHECK:STDOUT:   %.loc22_6.2: %empty_tuple.type = converted %.loc22_6.1, %empty_tuple [concrete = constants.%empty_tuple]
-// CHECK:STDOUT:   %F.call: init %empty_tuple.type = call %F.ref(%.loc22_6.2)
-// CHECK:STDOUT:   return
-// CHECK:STDOUT: }
-// CHECK:STDOUT:

toolchain/check/testdata/function/call/fail_runtime_implicit_param.carbon

@@ -12,7 +12,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/function/call/fail_runtime_implicit_param.carbon
 
-// CHECK:STDERR: fail_runtime_implicit_param.carbon:[[@LINE+4]]:6: error: implicit parameters of functions must be constant or `self` [ImplictParamMustBeConstant]
+// CHECK:STDERR: fail_runtime_implicit_param.carbon:[[@LINE+4]]:6: error: implicit parameters of functions must be constant [ImplictParamMustBeConstant]
 // CHECK:STDERR: fn F[s: ()]();
 // CHECK:STDERR:      ^~~~~
 // CHECK:STDERR:

toolchain/check/testdata/function/call/ref.carbon

@@ -31,7 +31,7 @@ library "[[@TEST_NAME]]";
 
 //@dump-sem-ir-begin
 class C {
-  fn F[ref self: Self]();
+  fn F(ref self);
 }
 
 fn G() {