[Serialization] Keep track of whether a module has an underlying Clang module.

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

include/swift/Serialization/ModuleFile.h

@@ -268,6 +268,10 @@ private:
     /// Whether this module file comes from a framework.
     unsigned IsFramework : 1;
 
+    /// Whether this module has a shadowed module that's part of its public
+    /// interface.
+    unsigned HasUnderlyingModule : 1;
+
     /// Whether or not ImportDecls is valid.
     unsigned ComputedImportDecls : 1;