[msan] Implement -msan-disable-checks.

To ease the deployment of KMSAN, we need a way to apply
__attribute__((no_sanitize("kernel-memory"))) to the whole source file.

Passing -msan-disable-checks=1 to the compiler will make it
treat every function in the file as if it was lacking the
sanitize_memory attribute.

Differential Revision: https://reviews.llvm.org/D115236