[tsan] Ignore memory accesses for libignored modules for "external" races

On Darwin, the setting ignore_noninstrumented_modules is used to suppress false positives in code that users don't have control of. The recently added "external" API (which can be used to detect races on objects provided by system libraries, but the race is actually user's fault) ignores this flag and it can report issues in non-instrumented modules. This patch fixes that.

Differential Revision: https://reviews.llvm.org/D31553

llvm-svn: 301000