[InterfacePrint] Enhance ASTPrinter to support type-specific interface printing.

When users try to print the interface of a specific type (most often through cursor
infor query of SourceKit), we should simplify the original decls by replacing
archetypes with instantiated types, hiding extension details, and omitting
unfulfilled extension requirements. So the users can get the straight-to-the-point
"type interface". This commit builds the testing infrastructure for this feature,
and implements the first trick that wraps extension contents into the interface body.

This commit also moves some generic testing support from SourceKit to Swift.

Swift SVN r32630

include/swift/Basic/PrefixMap.h

@@ -679,4 +679,4 @@ public:
 
 } // end namespace swift
 
-#endif
+#endif