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/interop/cpp/class/export/base.carbon

@@ -134,16 +134,16 @@ import Cpp;
 // CHECK:STDERR:
 abstract class Abstract {
   // CHECK:STDERR: fail_todo_abstract_nonvirtual_dtor_but_virtual_fns.carbon:[[@LINE+10]]:3: error: initialization of abstract type `Abstract` [AbstractTypeInInit]
-  // CHECK:STDERR:   abstract fn F[self: Self]();
-  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:   abstract fn F(self);
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~
   // CHECK:STDERR: fail_todo_abstract_nonvirtual_dtor_but_virtual_fns.carbon:[[@LINE-4]]:1: note: class was declared abstract here [ClassAbstractHere]
   // CHECK:STDERR: abstract class Abstract {
   // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~
   // CHECK:STDERR: fail_todo_abstract_nonvirtual_dtor_but_virtual_fns.carbon:[[@LINE+4]]:17: note: initializing function parameter [InCallToFunctionParam]
-  // CHECK:STDERR:   abstract fn F[self: Self]();
-  // CHECK:STDERR:                 ^~~~~~~~~~
+  // CHECK:STDERR:   abstract fn F(self);
+  // CHECK:STDERR:                 ^~~~
   // CHECK:STDERR:
-  abstract fn F[self: Self]();
+  abstract fn F(self);
 }
 
 inline Cpp '''

toolchain/check/testdata/interop/cpp/class/export/method.carbon

@@ -29,7 +29,7 @@ library "[[@TEST_NAME]]";
 import Cpp;
 
 class C {
-  fn M[self: Self]();
+  fn M(self);
 }
 
 inline Cpp '''
@@ -44,7 +44,7 @@ library "[[@TEST_NAME]]";
 import Cpp;
 
 class C {
-  fn M[self: Self]();
+  fn M(self);
 }
 
 inline Cpp '''
@@ -58,7 +58,7 @@ library "[[@TEST_NAME]]";
 import Cpp;
 
 class C {
-  fn M[ref self: Self]();
+  fn M(ref self);
 }
 
 inline Cpp '''
@@ -73,7 +73,7 @@ library "[[@TEST_NAME]]";
 import Cpp;
 
 class C {
-  fn M[ref self: Self]();
+  fn M(ref self);
 }
 
 inline Cpp '''
@@ -81,9 +81,9 @@ void F() {
   // CHECK:STDERR: fail_ref_method_rvalue.carbon:[[@LINE+7]]:3: error: 'this' argument to member function 'M' is an rvalue, but function has non-const lvalue ref-qualifier [CppInteropParseError]
   // CHECK:STDERR:    17 |   Carbon::C().M();
   // CHECK:STDERR:       |   ^
-  // CHECK:STDERR: fail_ref_method_rvalue.carbon:[[@LINE-8]]:25: note: 'M' declared here [CppInteropParseNote]
-  // CHECK:STDERR:     5 |   fn M[ref self: Self]();
-  // CHECK:STDERR:       |                         ^
+  // CHECK:STDERR: fail_ref_method_rvalue.carbon:[[@LINE-8]]:17: note: 'M' declared here [CppInteropParseNote]
+  // CHECK:STDERR:     5 |   fn M(ref self);
+  // CHECK:STDERR:       |                 ^
   // CHECK:STDERR:
   Carbon::C().M();
 }