Beef up some dllexport tests.

Adds some dllexport tests to verify that:
  - Variables in bss are exported appropriately
  - Non-dllexport symbols aliased to dllexport symbols are not exported
  - Symbols declared as dllexport but are not defined are not exported

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

Patch by Warren Ristow!

Differential Revision: http://reviews.llvm.org/D18682

llvm-svn: 265333