[Preprocessor] Support for '-dI' flag
Implement the -dI as supported by GCC: Output ‘#include’ directives in addition
to the result of preprocessing.
This change aims to add this option, pass it through to the preprocessor via
the options class, and when inclusions occur we output some information (+ test
Patch by Steve O'Brien!
Differential Revision: https://reviews.llvm.org/D26089