Fix spurious remarks when -Weverything is used.

With -Weverything, the backend remarks are enabled. This was
causing spurious diagnostics for remarks that we don't yet
handle (cf http://reviews.llvm.org/D3683).

This will stop being a problem once http://reviews.llvm.org/D3683
is committed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209823 91177308-0d34-0410-b5e6-96231b3b80d8