Implements CWG 1601 in [over.ics.rank/4.2]

Summary:
The overload resolution for enums with a fixed underlying type has changed in the C++14 standard. This patch implements the new rule.

Patch by Mark de Wever!

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65695

llvm-svn: 373866