swift234

Frontend no longer depends on ClangImporter. Weakly link the entry point.

This closes out <rdar://problem/14513108> -- sil-opt no longer depends on
ClangImporter or any Clang libraries. In theory, however, forcing a load
of a libSwiftClangImporter.dylib would allow sil-opt to process SIL files
containing Clang imports.

Swift SVN r6700