Previously the recommended pattern for having a "private" interface
for a public framework that was easy to strip out was to define an
explicit submodule named 'Private'. However, that makes it all too
easy to have dependencies from the public parts of the module on the
private parts, and other such problems. We're now recommending that
the private module map instead define a parallel module named with
"_Private" as a suffix. Clang already implemented this, but Swift
didn't get the same treatment.
This fix tries to avoid hardcoding too much; it notes that this kind
of remapping is limited to the name 'Private' as the second component
of a module hierarchy, but doesn't actually encode the mapping with the
"_Private" suffix.
rdar://problem/37553763
