swift234

[serialization] Reject loading a module with the wrong case.

Due to case-insensitive filesystems, "import foundation" can result in the
overlay module for Foundation being loaded. Everything is confused later on
because the (wrong) module name is used in manglings, leading to all sorts
of issues.

This is not the right fix for the problem, because a user really is allowed
to have modules named "foundation" and "FOUNDATION" and "Foundation" coexisting
on their system. To do that we'll want to check the actual case of a
.framework bundle or .swiftmodule file on disk and make sure it matches before
even trying to load the file. But this is a good sanity check anyway.

rdar://problem/15632996

Swift SVN r22818