Map structs and tuples to initializer lists in C++ overload resolution. (#6620)

When performing C++ overload resolution with an argument that is of
Carbon struct or tuple type, form a braced initializer list as the
placeholder argument. Note that this only affects overload resolution;
no new support for actually converting structs or tuples to C++ types is
added. In particular, while this does allow an empty class to be
initialized from `{}`, it does not allow a non-empty C++ class to be
initialized from a struct, as that is not yet supported in general.

---------

Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/check/cpp/import.cpp

@@ -2156,9 +2156,7 @@ static auto LookupMacro(Context& context, SemIR::NameScopeId scope_id,
   return nullptr;
 }
 
-// Gets the identifier info for a name. Returns `nullptr` if the name is not an
-// identifier name.
-static auto GetIdentifierInfo(Context& context, SemIR::NameId name_id)
+auto GetClangIdentifierInfo(Context& context, SemIR::NameId name_id)
     -> clang::IdentifierInfo* {
   std::optional<llvm::StringRef> string_name =
       context.names().GetAsStringIfIdentifier(name_id);
@@ -2183,7 +2181,8 @@ auto ImportNameFromCpp(Context& context, SemIR::LocId loc_id,
     return SemIR::ScopeLookupResult::MakeError();
   }
 
-  clang::IdentifierInfo* identifier_info = GetIdentifierInfo(context, name_id);
+  clang::IdentifierInfo* identifier_info =
+      GetClangIdentifierInfo(context, name_id);
   if (!identifier_info) {
     return SemIR::ScopeLookupResult::MakeNotFound();
   }