Handle and warn on aliases to weak aliases.

This produces valid IR now that llvm rejects aliases to weak aliases and warns
the user that the resolution is not changed if the weak alias is overridden.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204935 91177308-0d34-0410-b5e6-96231b3b80d8