Modules: Add -Rmodule-import

Add a remark for importing modules.  Depending on whether this is a
direct import (into the TU being built by this compiler instance) or
transitive import (into an already-imported module), the diagnostic has
two forms:

    importing module 'Foo' from 'path/to/Foo.pcm'
    importing module 'Foo' into 'Bar' from 'path/to/Foo.pcm'

Also drop a redundant FileCheck invocation in Rmodule-build.m that was
using -Reverything, since the notes from -Rmodule-import were confusing
it.

https://reviews.llvm.org/D58891

llvm-svn: 355477