Give incompatible function pointer warning its own diagnostic group
but still leave it as a subgroup of incompatible-pointer-types. This is in
preparation to promote -Wincompatible-function-pointer-types to error on
darwin.
Differential Revision: https://reviews.llvm.org/D22248
rdar://problem/12907612
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275907 91177308-0d34-0410-b5e6-96231b3b80d8
Bruno Cardoso Lopes