Fix Wbitfield-constant-conversion false positives

Summary:
The diagnostic did not handle ~ well. An expression such as ~0 is often used when 'all ones' is needed.

Differential Revision: https://reviews.llvm.org/D24232



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282156 91177308-0d34-0410-b5e6-96231b3b80d8