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

[Fangrui Song via cfe-commits]

MaskRay wrote:

https://discourse.llvm.org/t/rfc-nolock-and-noalloc-attributes/76837/75 contains a discussion about the subproject name.
Do you have opinions? @llvm/pr-subscribers-sanitizer @compnerd @petrhosek  @isanbard

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