Allow incomplete return types in function declarations. (#3875)

Move completeness check to the point where the function is defined or
first called. This means we also defer deciding whether the function has
a return slot until that point. Instead of storing a return slot per
function, store the location of the return storage, which may or may not
be used, and compute and store a separate flag saying whether to use it
at the point of first use or definition.

This is the final piece in supporting simple `Make` functions in classes
as a replacement for constructors.

toolchain/check/context.cpp

@@ -627,7 +627,9 @@ auto Context::FinalizeGlobalInit() -> void {
          .implicit_param_refs_id = SemIR::InstBlockId::Empty,
          .param_refs_id = SemIR::InstBlockId::Empty,
          .return_type_id = SemIR::TypeId::Invalid,
-         .return_slot_id = SemIR::InstId::Invalid,
+         .return_storage_id = SemIR::InstId::Invalid,
+         .is_extern = false,
+         .return_slot = SemIR::Function::ReturnSlot::Absent,
          .body_block_ids = {SemIR::InstBlockId::GlobalInit}});
   } else {
     inst_block_stack().PopGlobalInit();