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.

lib/AST/GenericSignature.cpp

@@ -1144,3 +1144,9 @@ unsigned GenericParamKey::findIndexIn(
   // We didn't find the parameter we were looking for.
   return genericParams.size();
 }
+
+unsigned GenericSignature::getGenericParamOrdinal(GenericTypeParamType *param) {
+  return GenericParamKey(param->getDepth(), param->getIndex())
+    .findIndexIn(getGenericParams());
+}
+