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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

We talked about this PR a bit in my office hours yesterday and one thing which came up for feedback to WG14 is that this should be spelled `_Defer` and not `defer` if the committee is serious about the feature (which I believe they are). It's going into a TS, and so users have to opt-in for `defer` to become a keyword which may break their code. That's fine and we can live with it. But when `defer` is rolled into the IS, that makes for a harder migration path. But more importantly, we like this feature -- we want to expose it as a conforming extension in other language modes; that's not possible with the keyword spelled `defer` because that'd be a non-conforming extension. And from looking at available open source code, the identifier `defer` is used quite frequently.

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