swift234

[Driver] If lib/arc/ isn't present next to Swift, look next to Xcode's Clang.

Deploying to older OSs requires linking in a compatibility library called
"arclite", but this library isn't open source and won't be distributed with
our open source downloads. Fall back to the version in Xcode.

The next step is to remove the local symlink used in builds, but I wanted to
handle that separately.

rdar://problem/23421436