[MSan] Add functions to enable/disable interceptor checks.

Summary:
The motivation for this change is to make libFuzzer+MSan possible
without instrumenting libFuzzer.

See https://github.com/google/sanitizers/issues/958.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D48890

llvm-svn: 336447