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/cpp/export.cpp

@@ -373,23 +373,22 @@ struct FunctionInfo {
         decl_context(decl_context) {
     auto function_params =
         context.inst_blocks().Get(function.call_param_patterns_id);
+    const auto& ranges = function.call_param_ranges;
+    auto explicit_begin = ranges.explicit_begin().index;
 
-    // Get the function's `self` parameter, if present.
-    if (function.call_param_ranges.implicit_size() > 0) {
-      CARBON_CHECK(function.call_param_ranges.implicit_size() == 1);
-
-      auto param_inst_id =
-          function_params[function.call_param_ranges.implicit_begin().index];
-      self_param = Param(context, param_inst_id);
+    // Get the function's `self` parameter, if present. `self` is the first
+    // explicit call parameter. (The lowered call parameters are leaf patterns
+    // without binding names, so we rely on `self_param_id` and the positional
+    // convention rather than inspecting the pattern.)
+    if (function.self_param_id.has_value()) {
+      CARBON_CHECK(explicit_begin != ranges.explicit_end().index);
+      self_param = Param(context, function_params[explicit_begin]);
+      ++explicit_begin;
     }
 
-    // Get the function's explicit parameters.
-    function_params =
-        function_params.drop_front(function.call_param_ranges.implicit_size());
-    function_params =
-        function_params.drop_back(function.call_param_ranges.return_size());
-    for (auto param_inst_id : function_params) {
-      explicit_params.push_back(Param(context, param_inst_id));
+    // The remaining explicit parameters are the caller-provided arguments.
+    for (auto i = explicit_begin; i != ranges.explicit_end().index; ++i) {
+      explicit_params.push_back(Param(context, function_params[i]));
     }
   }