[tsan] Add a test for "external" API that checks the dup suppression is based on the caller PC

We need to make sure that the "external" API isn't dup'ing all data races into a single one (because the stack might look the same) and suppressing all external races. This works now, so just adding a test for that.

Differential Revision: https://reviews.llvm.org/D31734

llvm-svn: 301011