[Clang importer] Basic import-as-member inference system

Introduces import-as-member (IAM) inferene system, to automatically
infer details that would otherwise have to be manually specified with
the swift_name attribute.

Basic functionality present, though there are some issues with
properties at the moment. No hooks, options, or tests yet, stay tuned.

include/swift/Basic/StringExtras.h

@@ -182,6 +182,16 @@ namespace swift {
       unsigned getPosition() const {
         return Position;
       }
+
+      /// Retrieve the string up until this iterator
+      StringRef getPriorStr() const {
+        return String.slice(0, Position);
+      }
+
+      /// Retrieve the rest of the string (including this position)
+      StringRef getRestOfStr() const {
+        return String.slice(Position, String.size());
+      }
     };
 
     /// Find the first camelCase word in the given string.