In case `@import Foo.Private` fails because the submodule doesn't exist,
look for `Foo_Private` (if available) and build/load that module
instead. In that process emit a warning and tell the user about the
assumption.
The intention here is to assist all existing private modules owners
(in ObjC and Swift) to migrate to the new `Foo_Private` syntax.
rdar://problem/36023940
llvm-svn: 321342
Bruno Cardoso Lopes