[clang] [compiler-rt] [compiler-rt] Realtime Sanitizer: Introduce Realtime Sanitizer (RTSan) backend (PR #92460)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> This is PR 1/?? (3? 4?) introducing the main guts of the realtime sanitizer. For more information, please see the [discourse thread](https://discourse.llvm.org/t/rfc-nolock-and-noalloc-attributes/76837). We have also put together a [reviewer support document](https://github.com/realtime-sanitizer/radsan/blob/doc/review-support/doc/review.md?rgh-link-date=2024-05-16T21%3A34%3A46Z) to show what our intention is.

clangDriver changes are usually the last. The expectation is that if `-fsanitize=realtime` does not return an error, there should be some basic functionality.

* clangCodeGen , testing with `%clang_cc1`
* clangDriver, testing with `%clang`

We typically split clangDriver and compiler-rt changes. Runtime compiler-rt tests often expose minor issues that only fail on certain platforms. Keeping it separate prevents us from potentially reverting the compiler-rt code, leading to unnecessary churn.
The recent numerical stability sanitizer did not do a good job for this aspect. I apologize for that as a reviewer.)

https://github.com/llvm/llvm-project/pull/92460