This allows plugins which add AST passes to also define pragmas to do things
like only enable certain behaviour of the AST pass in files where a certain
pragma is used.

Differential Revision: http://reviews.llvm.org/D18319


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@265295 91177308-0d34-0410-b5e6-96231b3b80d8