Restore impl of DerivedRecordSize in RDAT (#4842)

This default method implementation is necessary for the base->derived size version mechanism to work.
This is only specialized when a base has a derived, and it relies on the newest version to use the default implementation (no specialization).

The implementation was removed in commit 8ee6ed1.

Author: tex3d

include/dxc/DxilContainer/DxilRuntimeReflection.h

@@ -249,7 +249,7 @@ class RecordTraits {
   // RecordSize() is defined in order to allow for use of forward decl type in RecordRef
   static constexpr size_t RecordSize() { return sizeof(_T); }
   static constexpr size_t MaxRecordSize() { return RecordTraits<_T>::DerivedRecordSize(); }
-  static constexpr size_t DerivedRecordSize();
+  static constexpr size_t DerivedRecordSize() { return sizeof(_T); }
 };
 
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