Insert interception point onStartOfTranslationUnit.

Often users of the ASTMatchers want to add tasks that are done once per
translation unit, for example, cleaning up caches. Combined with the
interception point for the end of source file one can add to the factory
creation, this covers the cases we've seen users need.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167271 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed