[Modules] Map missing private submodules from Foo.Private to Foo_Private

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321342 91177308-0d34-0410-b5e6-96231b3b80d8