[Type checker] Requestify the formation of an abstract generic signature

Introduce a request to form an abstract generic signature given a
base signature, additional generic parameters, and additional
requirements. It is meant to provide a caching layer in front of the
generic signature builder.

Switch one direct client of the generic signature builder over to this
mechanism, the formation of a generic signature for an existential
type.

include/swift/Basic/SimpleDisplay.h

@@ -121,6 +121,20 @@ namespace swift {
     }
     out << "}";
   }
+
+  template<typename T>
+  void simple_display(llvm::raw_ostream &out,
+                      const llvm::SmallVectorImpl<T> &vec) {
+    out << "{";
+    bool first = true;
+    for (const T &value : vec) {
+      if (first) first = false;
+      else out << ", ";
+
+      simple_display(out, value);
+    }
+    out << "}";
+  }
 }
 
 #endif // SWIFT_BASIC_SIMPLE_DISPLAY_H