Record whether a module came from a private module map

Record whether a module came from a private module map

Right now we only use this information in one place, immediately after
we calculate it, but it's still nice information to have. The Swift
project is going to use this to tidy up its "API notes" feature (see
past discussion on cfe-dev that never quite converged).

Reviewed by Bruno Cardoso Lopes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@330452 91177308-0d34-0410-b5e6-96231b3b80d8