[clang] [clang][Parse] Fix __extension__ not silencing diagnostics (PR #221623)

[Shengxin Pei via cfe-commits]

TPPPP72 wrote:

> @TPPPP72 Have not posted an issue and haven't found an existing one. Happy to open one

Perhaps you could add more background information to your PR description; that would make it easier for us to understand your changes. There is no need to open a separate issue.


https://github.com/llvm/llvm-project/pull/221623