Improve CHECK-NOT robustness of dllexport tests

This changes some dllexport tests, to verify that some symbols that
should not be exported are not, in a way that improves the robustness
of CHECK-SAME interaction with CHECK-NOT.

We plan to enable dllimport/dllexport support for the PS4, and these
changes are for points we noticed in our internal testing.

Patch by Warren Ristow!

llvm-svn: 265106