IDE: refactor USR generation to request evaluator model.

We refactor the existing USR generation code to use the request evaluator
model. Most part of this patch is NFC, however a slight difference is
using this model allows us to cache the calculated USRs in the evaluator.

include/swift/Basic/SimpleDisplay.h

@@ -49,6 +49,7 @@ namespace swift {
   HAS_TRIVIAL_DISPLAY(float);
   HAS_TRIVIAL_DISPLAY(double);
   HAS_TRIVIAL_DISPLAY(bool);
+  HAS_TRIVIAL_DISPLAY(std::string);
 
 #undef HAS_TRIVIAL_DISPLAY