-frewrite-imports: Add support for wildcard rules in umbrella modules with

This trips over a few other limitations, but in the interests of incremental development I'm starting here & I'll look at the issues with -verify and filesystem checks (the fact that the behavior depends on the existence of a 'foo' directory even though it shouldn't need it), etc.

Reviewers: rsmith

Differential Revision: https://reviews.llvm.org/D61656

llvm-svn: 360195