Add a plugin testcase for merging weak variables.

I initially thought I could implement COMDATs with aliases by just
internalizing GVs instead of dropping them. This is a counter
example: Internalizing one of the @a would make @b and @c point
to different variables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215447 91177308-0d34-0410-b5e6-96231b3b80d8