[testsuite] Remove testing failures vestiges.

Some tests are failing on macOS when building with the in-tree
clang, and this is because they're conditional on the version released.
Apple releases using a different versioning number, but as these are
conditional on clang < 7, they fail for clang ToT (which is 6.0).
As a general solution, we actually need either a mapping between
Apple internal release version and public ones.

That said, I discussed this with Fred , and Apple Clang 6.0 seems
to be old enough that we can remove this altogether (which means I
can delay implementing the general purpose solution for a bit).

Differential Revision:  https://reviews.llvm.org/D41101

llvm-svn: 320444