[Modules] Extend -fmodule-name semantic for frameworks with private modules

Assume Foo.framework with two module maps and two modules Foo and
Foo_Private.

Framework authors need to skip building both Foo and Foo_Private when
using -fmodule-name=Foo, since both are part of the framework and used
interchangeably during compilation.

rdar://problem/37500098

llvm-svn: 325305