Place bitfield -Wconstant-conversion warning into subgroup called -Wbitfield-constant-conversion.

This is to just allow more precise diagnostic control.

Implements <rdar://problem/13766026>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180773 91177308-0d34-0410-b5e6-96231b3b80d8