clang -extract-api should accept multiple headers and forward them to a
single CC1 instance. This change introduces a new ExtractAPIJobAction.
Currently API Extraction is done during the Precompile phase as this is
the current phase that matches the requirements the most. Adding a new
phase would need to change some logic in how phases are scheduled. If
the headers scheduled for API extraction are of different types the
driver emits a diagnostic.
Differential Revision: https://reviews.llvm.org/D121936
Daniel Grumberg