[Index] Stopped wrapping FrontendActions in libIndex and its users

Exposed a new function, createIndexingASTConsumer, that creates an
ASTConsumer. ASTConsumers compose well.

Removed wrapping functionality from createIndexingAction.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@370337 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed