swift234

Re-apply "[serialization] Reject loading a module with the wrong case."

This included a test that failed on case-sensitive filesystems. Test fixed.

(Aside: Why not just have this fail with "no such module"? Why use a different
error? Because even if "import FOO" picks up a module named 'Foo', there may
actually be a module named 'FOO' on the system (in another folder), and we
should be able to find that. Fixing that is tracked by rdar://problem/18691936.)

rdar://problem/15632996 (again)

Swift SVN r22856