Remove qualtype qualifier in coroutine error to prevent assert in debug

[tks2103]

Summary

A forward-declared coroutine_traits should trip an error; we need
a complete type.

Unfortunately, in debug mode only, we trip an assert when attempting
to provide the fully qualified type for the error message.
If you try to compile a program with a forward-declared
coroutine_traits in debug mode, clang will crash.

I've included a test for the behavior and removed the q modifier
on the error message. This prevents the crash in debug mode and
does not change the behavior for the error message on a
forward-declaration of a coroutine_traits type.

Test Plan:
I've included a test for the forward-declaration.

[modocache]

Excellent, thanks!

[modocache]

If you don't have commit access, let me know here if you'd like me to commit this on your behalf.

[tks2103]

@modocache please commit on my behalf please =)

include/clang/Basic/DiagnosticSemaKinds.td

 def err_implied_std_coroutine_traits_promise_type_not_found : Error<
   "this function cannot be a coroutine: %q0 has no member named 'promise_type'">;
 def err_implied_std_coroutine_traits_promise_type_not_class : Error<
   "this function cannot be a coroutine: %0 is not a class">;
 def err_coroutine_promise_type_incomplete : Error<
   "this function cannot be a coroutine: %0 is an incomplete type">;
 def err_coroutine_type_missing_specialization : Error<
   "this function cannot be a coroutine: missing definition of "
-  "specialization %q0">;
+  "specialization %0">;
 def err_coroutine_promise_incompatible_return_functions : Error<
   "the coroutine promise type %0 declares both 'return_value' and 'return_void'">;
 def err_coroutine_promise_requires_return_function : Error<
   "the coroutine promise type %0 must declare either 'return_value' or 'return_void'">;
 def note_coroutine_promise_implicit_await_transform_required_here : Note<
   "call to 'await_transform' implicitly required by 'co_await' here">;
 def note_coroutine_promise_suspend_implicitly_required : Note<
   "call to '%select{initial_suspend|final_suspend}0' implicitly "

test/SemaCXX/coroutine-traits-undefined-template.cpp

+// test/SemaCXX/coroutine-traits-undefined-template.cpp
+
+// This file contains references to sections of the Coroutines TS, which can be
+// found at http://wg21.link/coroutines.
+
+// RUN: %clang_cc1 -std=c++14 -fcoroutines-ts -verify %s -fcxx-exceptions -fexceptions -Wunused-result
+
+namespace std {
+namespace experimental {
+
+template<typename ...T>
+struct coroutine_traits {
+  struct promise_type {};
+};
+
+template<> struct coroutine_traits<void>; // expected-note {{forward declaration of 'std::experimental::coroutine_traits<void>'}}
+}} // namespace std::experimental
+
+void uses_forward_declaration() {
+  co_return; // expected-error {{this function cannot be a coroutine: missing definition of specialization 'coroutine_traits<void>'}}
+}