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
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