swift234

[ClangImporter] Use Clang's -fembed-bitcode mode when under -embed-bitcode.

This should have no functionality change, but is supposed to keep us from
accidentally relying on the "full" Clang importer when in a backend job.
I tested it by archiving a little iOS app from a developer.

Unfortunately, part of the motivation here was that we'd get error messages when
we pass something Clang doesn't like, and that doesn't seem to be happening.

rdar://problem/21389553

Swift SVN r30407