[clang] [llvm] [Clang] Add __builtin_allow_sanitize_check() (PR #172030)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

(Response delayed because I was off last two weeks.)

I don't think the name of the intrinsic makes sense.  Can we just call it `__builtin_is_sanitizer_enabled()`?

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I think I'd like a Discourse post so people are aware we've added this. I'm not expecting any significant changes to the design, though, because there's basically only one way to do this.

https://github.com/llvm/llvm-project/pull/172030