[clang] Surface error for plain return statement in coroutine earlier (PR #100985)

[Ilya Biryukov via cfe-commits]

================
@@ -291,6 +294,38 @@ void mixed_coreturn_template2(bool b, T) {
     return; // expected-error {{not allowed in coroutine}}
 }
 
+struct promise_handle;
+
+struct Handle : std::coroutine_handle<promise_handle> { // expected-note 2{{candidate constructor (the implicit copy constructor) not viable}}
+    // expected-note at -1 2{{candidate constructor (the implicit move constructor) not viable}}
+    using promise_type = promise_handle;
+};
+
+struct promise_handle {
+    Handle get_return_object() noexcept {
+      { return Handle(std::coroutine_handle<Handle::promise_type>::from_promise(*this)); }
+    }
+    suspend_never initial_suspend() const noexcept { return {}; }
+    suspend_never final_suspend() const noexcept { return {}; }
+    void return_void() const noexcept {}
+    void unhandled_exception() const noexcept {}
+};
+
+Handle mixed_return_value() {
+  co_await a; // expected-note {{function is a coroutine due to use of 'co_await' here}}
+  return 0; // expected-error {{return statement not allowed in coroutine}}
+  // expected-error at -1 {{no viable conversion from returned value of type}}
----------------
ilya-biryukov wrote:

the `-verify` and `expected-error` does not actually let us check the order in which the errors are reported.

Could we add another test file that would do something like:

```
// RUN: %clang_cc1 -fsyntax-only %s 2>& 1 | FileCheck
// The error about a function not being a coroutine is usually more relevant than the conversion error.
// Check that we show it first.
<test code here>
// CHECK-NOT: cannot convert
// CHECK: return statement not allowed in coroutine
// CHECK: cannot convert
```
it does not have to be detailed, just to make sure we actually managed to keep the errors in that order

https://github.com/llvm/llvm-project/pull/100985