Create a new 'flag_enum' attribute.

This attribute serves as a hint to improve warnings about the ranges of
enumerators used as flag types. It currently has no working C++ implementation
due to different semantics for enums in C++. For more explanation, see the docs
and testcases.

Reviewed by Aaron Ballman.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222906 91177308-0d34-0410-b5e6-96231b3b80d8