[libc++][PSTL] Disable `-Wpass-failed`.

PSTL contains many pragmas that request loop vectorization, which would
produce a warning when the compiler is unable to fulfill the request (if
`-Wpass-failed` is enabled). This is normal and expected in some cases,
and we don't want `-Werror` to turn that into a compilation failure.

Differential Revision: https://reviews.llvm.org/D157145

(cherry picked from commit 1d340250)