[PCH] Temporarily disable the "ambiguous macro" warning that is currently bogus with a PCH
that redefined a macro without undef'ing it first.

Proper reconstruction of the macro info history from modules will properly fix this in subsequent commits.

rdar://13016031

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173281 91177308-0d34-0410-b5e6-96231b3b80d8