Add escape hatches for source-breaking throughput improvements (#3027)

stl/inc/functional

@@ -14,6 +14,9 @@
 #include <xmemory>
 #include <xstddef>
 #if _HAS_CXX17
+#ifdef _LEGACY_CODE_ASSUMES_FUNCTIONAL_INCLUDES_MEMORY
+#include <memory>
+#endif // _LEGACY_CODE_ASSUMES_FUNCTIONAL_INCLUDES_MEMORY
 #include <unordered_map>
 #endif // _HAS_CXX17
 #ifdef __cpp_lib_concepts

stl/inc/xhash

@@ -90,6 +90,11 @@ namespace stdext {
         _Pr comp{}; // the comparator object
     };
 } // namespace stdext
+
+_STD_BEGIN
+using stdext::hash_compare; // Non-Standard, for legacy source compatibility
+_STD_END
+
 #endif // _SILENCE_STDEXT_HASH_DEPRECATION_WARNINGS
 
 _STD_BEGIN