[libclang 6/8] Add support for reading implicit attributes

Summary:
Having access to implicit attributes is sometimes useful so users of libclang don't have to duplicate some of the logic in sema.

This depends on D49081 since it also adds a CXTranslationUnit flag.

Reviewers: yvvan, jbcoe

Reviewed By: yvvan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49631

llvm-svn: 338815