[clang-format] Recognize c++ coroutine keywords as unary operator to avoid...

[clang-format] Recognize c++ coroutine keywords as unary operator to avoid misleading pointer alignment

Summary: The clang-format may go wrong when handle c++ coroutine keywords and pointer.
The default value for PointerAlignment is PAS_Right. So the following format is good:
```
co_return *a;
```
But within some code style, the value for PointerAlignment is PAS_Left, the behavior goes wrong:
```
co_return* a;
```

test-plan: check-clang

reviewers: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D91245