[libclang 8/8] Add support for the flag_enum attribute

Summary:
This adds support to libclang for reading the flag_enum attribute.

This also bumps CINDEX_VERSION_MINOR for this patch series.

Reviewers: yvvan, jbcoe

Reviewed By: yvvan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49635

llvm-svn: 338820