swift234

[ClangImporter] Prefer -Xcc options, then -F search paths, then -I paths.

This makes Swift (a) more likely to prefer frameworks over bare headers,
reducing potential issues with non-modular headers, and (b) more likely
to fail in the same way as LLDB if the -Xcc options also contain or affect
search paths.

rdar://problem/22413525

Swift SVN r31950