cpp11-migrate: Add a class to support include directives modifications

The IncludeDirectives class helps with detecting and modifying #include
directives. For now it allows the users to add angled-includes in a source file.
This is a start for this class that will evolve in the future to add more
functionality.

This should fix the reverted commit r188791 (buildbot failures on Windows).

llvm-svn: 189017