[clang] [coroutine] Suppress unreachable-code warning on coroutine statements. (PR #77454)

[Utkarsh Saxena via cfe-commits]

================
@@ -60,6 +61,45 @@ static bool isTrivialDoWhile(const CFGBlock *B, const Stmt *S) {
   return false;
 }
 
+// Check if the block starts with a coroutine statement and see if the given
+// unreachable 'S' is the substmt of the coroutine statement.
+//
+// We suppress the unreachable warning for cases where an unreachable code is
+// a substmt of the coroutine statement, becase removing it will change the
+// function semantic if this is the only coroutine statement of the coroutine.
+static bool isInCoroutineStmt(const CFGBlock *Block, const Stmt* S) {
----------------
usx95 wrote:

I do not think this works for more interesting cases when coroutine statements are before the `abort`.
Eg: 
```cpp
task test() {
  co_await 1;
  abort();
  co_await 2; // expected-warning {{code will never be executed}}
}
```
would fail. We would want a warning in this case.
`co_await 2` has its own block which would be passed to this function (B1 in [CFG](https://godbolt.org/z/Ed3hhzce9))

More test ideas:
```cpp
bool foo();
task test() {
  if (foo()) {
    std::abort();
    co_await 1; // expected-warning {{code will never be executed}}
  }
  co_await 2;
}
```

https://github.com/llvm/llvm-project/pull/77454