Replace nominal type descriptors with a hierarchy of context descriptors.

This new format more efficiently represents existing information, while
more accurately encoding important information about nested generic
contexts with same-type and layout constraints that need to be evaluated
at runtime. It's also designed with an eye to forward- and
backward-compatible expansion for ABI stability with future Swift
versions.

include/swift/Basic/RelativePointer.h

@@ -490,6 +490,10 @@ public:
   IntTy getInt() const & {
     return IntTy(RelativeOffsetPlusInt & getMask());
   }
+  
+  Offset getOpaqueValue() const & {
+    return RelativeOffsetPlusInt;
+  }
 };
 
 // Type aliases for "far" relative pointers, which need to be able to reach