Make clang-tidy's runCheckOnCode actually use the DiagnosticConsumer.

A precondition of that was to run both the preprocessor checks and AST
checks from the same FrontendAction, otherwise we'd have needed to
duplicate all involved objects in order to not have any references to a
deleted source manager.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@219212 91177308-0d34-0410-b5e6-96231b3b80d8