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/import/abstract.carbon

@@ -46,7 +46,7 @@ import Cpp library "abstract.h";
 
 class C {
   extend base: Cpp.A;
-  override fn f[unused ref self: Self]() {}
+  override fn f(unused ref self) {}
 }
 
 // --- fail_impl_mismatch_member.carbon
@@ -58,10 +58,10 @@ import Cpp library "abstract.h";
 class C {
   extend base: Cpp.A;
   // CHECK:STDERR: fail_impl_mismatch_member.carbon:[[@LINE+4]]:3: error: override without compatible virtual in base class [OverrideWithoutVirtualInBase]
-  // CHECK:STDERR:   override fn invalid[unused ref self: Self]() {}
-  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:   override fn invalid(unused ref self) {}
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   // CHECK:STDERR:
-  override fn invalid[unused ref self: Self]() {}
+  override fn invalid(unused ref self) {}
 }
 
 // --- abstract_final.h

toolchain/check/testdata/interop/cpp/class/import/access.carbon

@@ -127,7 +127,7 @@ import Cpp library "non_function_member_protected.h";
 
 class Derived {
   extend base: Cpp.C;
-  fn F[self: Self]() {
+  fn F(self) {
     //@dump-sem-ir-begin
     let unused instance_data: i32 = self.instance_data;
     let unused unqualified_static_data: i32 = static_data;
@@ -182,7 +182,7 @@ import Cpp library "non_function_member_private.h";
 
 class Derived {
   extend base: Cpp.C;
-  fn F[self: Self]() {
+  fn F(self) {
     // CHECK:STDERR: fail_import_non_function_member_private_extend.carbon:[[@LINE+8]]:37: error: cannot access private member `instance_data` of type `Cpp.C` [ClassInvalidMemberAccess]
     // CHECK:STDERR:     let unused instance_data: i32 = self.instance_data;
     // CHECK:STDERR:                                     ^~~~~~~~~~~~~~~~~~
@@ -313,7 +313,7 @@ import Cpp library "function_member_protected.h";
 
 class Derived {
   extend base: Cpp.C;
-  fn F[self: Self]() {
+  fn F(self) {
     //@dump-sem-ir-begin
     // TODO: It should be possible to call protected methods in the base class.
     // CHECK:STDERR: fail_todo_import_function_member_protected_extend_call_instance_in_member.carbon:[[@LINE+8]]:22: error: no matching function for call to 'instance_fn' [CppInteropParseError]
@@ -337,7 +337,7 @@ import Cpp library "function_member_protected.h";
 
 class Derived {
   extend base: Cpp.C;
-  fn F[unused self: Self]() {
+  fn F(unused self) {
     //@dump-sem-ir-begin
     static_fn();
     Derived.static_fn();
@@ -366,7 +366,7 @@ import Cpp library "function_member_private.h";
 
 class Derived {
   extend base: Cpp.C;
-  fn F[self: Self]() {
+  fn F(self) {
     // CHECK:STDERR: fail_import_function_member_private.carbon:[[@LINE+7]]:5: error: cannot access private member `instance_fn` of type `Cpp.C` [ClassInvalidMemberAccess]
     // CHECK:STDERR:     self.instance_fn();
     // CHECK:STDERR:     ^~~~~~~~~~~~~~~~

toolchain/check/testdata/interop/cpp/class/import/dynamic.carbon

@@ -39,7 +39,7 @@ auto Use(Base& b) -> int {
 
 base class Derived {
   extend base: Cpp.Base;
-  virtual fn other_func[unused self: Self]() -> i32 {
+  virtual fn other_func(unused self) -> i32 {
     return 3;
   }
 }