swift234

Put the current target into LangOptions.

This has been long in coming. We always had it in IRGenOpts (in string form).
We had the version number in LangOpts for availability purposes. We had to
pass IRGenOpts to the ClangImporter to actually create the right target.
Some of our semantic checks tested the current OS by looking at the "os"
target configuration! And we're about to need to serialize the target for
debugging purposes.

Swift SVN r24468