[Parser] Improve diagnostic and error recovery when C++ keywords are used as identifiers.

Summary:
Previously, clang emitted a less-usefull diagnostic and didnt recover
well when the keywords is used as identifier in function paramter.

```
void foo(int case, int x); // previously we drop all parameters after
`int case`.
```

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77633