[compiler-rt] [compiler-rt][rtsan] Fix build failures when building against in-tree libc++ (PR #175638)

[Nick Hutchinson via llvm-commits]

nickhutchinson wrote:

@cjappl @davidtrevelyan Thanks for the reviews. Are you able to merge this? I do not have commit access.

https://github.com/llvm/llvm-project/pull/175638