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
Haojian Wu