[Diagnostics] Warn if enumeration type mismatch in conditional expression

Summary:
- Useful warning
- GCC compatibility (GCC warns in C++ mode)

Reviewers: rsmith, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67919

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373252 91177308-0d34-0410-b5e6-96231b3b80d8