If the user has requested this warning, we should emit it, even if it's not
an extension in the current language mode. However, being an extension is
more important, so prefer the pedantic warning or the pedantic-compatibility
warning if those are enabled.

<rdar://problem/12922063>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189110 91177308-0d34-0410-b5e6-96231b3b80d8