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.

stdlib/public/runtime/ErrorDefaultImpls.cpp

@@ -32,7 +32,7 @@ intptr_t _swift_stdlib_getDefaultErrorCode(OpaqueValue *error,
       // Enum tags use negative values for payload cases, so adjust code to be
       // in the range [0, num-cases).
       result = T->vw_getEnumTag(error) +
-        T->getNominalTypeDescriptor()->Enum.getNumPayloadCases();
+        T->getTypeContextDescriptor()->Enum.getNumPayloadCases();
       break;
 
     case MetadataKind::Class: