Previously, we depended on whether or not a serialized module was located
within a framework bundle to consider whether or not it may have a "Clang
half". However, LLDB loads serialized modules from dSYM bundles. Rather
than try to figure out if such a module is "really" a framework, just track
whether the original module was built with -import-underlying-module. If so,
consider the underlying Clang module to be re-exported.
rdar://problem/18099523
Swift SVN r21544
