Add -iapinotes-modules search path option.
This option adds a new module-centric search path to find the API
notes file that applies to the current module that needs to be
built. It costs us only 2 stats per search path at module construction
time to determine if API notes are available, making it far more
efficient (and easier to use) than the prior API notes searching
mechanism. It also fits much better with the future direction of the
Swift Clang importer, which will soon delegate its responsibilities to
Clang's API notes infrastructure.
This is part of rdar://problem/24447420.