Split "discards qualifiers" warnings of -Wincompatible-pointer-types into subgroup.

This allows users to promote -Wincompatible-pointer-type warnings to
errors but keep those for "discard qualifiers" as warnings (if they
so desire).

Addresses <rdar://problem/13062738>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173184 91177308-0d34-0410-b5e6-96231b3b80d8