[Sema] Create a separate group for incompatible function pointer warning

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

llvm-svn: 275907