AsmParser: add a warning for compatibility parsing

This adds a warning when linker_private or linker_private_weak is provided and
we handle it in a compatible manner.

Suggested by Chris Lattner!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205681 91177308-0d34-0410-b5e6-96231b3b80d8