[clang] [clang-tools-extra] [Clang] Add support for the C `defer` TS (PR #162848)

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Sirraide wrote:

> I recommend we only support `_Defer` as a keyword and let users include `<stddefer.h>` to get the prettier spelling. 

Sgtm

> I think we should vend that header file ourselves instead of leaving it to a C library

Yeah, definitely.



https://github.com/llvm/llvm-project/pull/162848