clang-cl /fallback: turn the note into a warning

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201626 91177308-0d34-0410-b5e6-96231b3b80d8
Remove dead code.

Clang never produces a linker private object, so this code is dead.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201627 91177308-0d34-0410-b5e6-96231b3b80d8