Accept #pragma warning(push, 0) without warning

This partially addresses PR17435, but it doesn't actually implement the
pragma.  If we implement it, we should map levels 1-4 to something like
-Wall and level 0 to something like -w.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@191833 91177308-0d34-0410-b5e6-96231b3b80d8