Add support for -> auto deduced return types. (#7931)

The return type is deduced from the function's `return` statements
(currently required to all have the same type), or from its first
`returned var`. Calls to generic functions with deduced return types are
not yet supported.

When a function has a deduced return type, we defer the return type
portion of the callee pattern match until deduction, and during
deduction we extend the decl block and the pattern block with the
return-related instructions.

Conversions for `return expr;` statements are deferred until the return
type has been deduced. The design requires that we take the common type
of the returned types as the deduced return type, so we can't in general
perform any conversions for returned expressions until we reach the end
of the function. This is accomplished by inserting a `splice_block`
where the conversions will go, and later updating that instruction to
contain the conversion.

Assisted-by: Claude Opus 5.5 via Antigravity

toolchain/check/scope_stack.cpp

@@ -26,6 +26,8 @@ auto ScopeStack::sem_ir() const -> const SemIR::File& {
 
 auto ScopeStack::VerifyOnFinish() const -> void {
   CARBON_CHECK(return_scope_stack_.empty(), "{0}", return_scope_stack_.size());
+  CARBON_CHECK(pending_return_stack_.empty(), "{0}",
+               pending_return_stack_.all_values_size());
   CARBON_CHECK(break_continue_stack_.empty(), "{0}",
                break_continue_stack_.size());
   CARBON_CHECK(scope_stack_.empty(), "{0}", scope_stack_.size());
@@ -136,6 +138,7 @@ auto ScopeStack::PushForFunctionBody(SemIR::InstId scope_inst_id) -> void {
 
   return_scope_stack_.push_back(
       {.decl_id = scope_inst_id, .cleanup_scope_depth = cleanup_scope_depth()});
+  pending_return_stack_.PushArray();
 }
 
 auto ScopeStack::Pop(bool check_unused) -> void {
@@ -172,6 +175,9 @@ auto ScopeStack::Pop(bool check_unused) -> void {
 
     if (return_scope_stack_.back().decl_id == scope.scope_inst_id) {
       // Leaving the function scope.
+      CARBON_CHECK(pending_return_stack_.PeekArray().empty(),
+                   "Leaving function with pending `return`s");
+      pending_return_stack_.PopArray();
       return_scope_stack_.pop_back();
     } else {
       if (return_scope_stack_.back().nested_scope_index == scope.index) {