Don't add swift_retainCount() to RuntimeFunctions.def, but explicitly link it in linkEmbeddedRuntimeFromStdlib()

include/swift/Runtime/RuntimeFunctions.def

@@ -204,14 +204,6 @@ FUNCTION(CopyPOD, swift_copyPOD, C_CC, AlwaysAvailable,
          EFFECT(NoEffect),
          UNKNOWN_MEMEFFECTS)
 
-// size_t swift_retainCount(void *ptr);
-FUNCTION(NativeRetainCount, swift_retainCount, C_CC, AlwaysAvailable,
-         RETURNS(SizeTy),
-         ARGS(RefCountedPtrTy),
-         ATTRS(NoUnwind, WillReturn),
-         EFFECT(RefCounting),
-         UNKNOWN_MEMEFFECTS)
-
 // void *swift_retain(void *ptr);
 FUNCTION(NativeStrongRetain, swift_retain, C_CC, AlwaysAvailable,
          RETURNS(RefCountedPtrTy),

lib/SILOptimizer/UtilityPasses/Link.cpp

@@ -63,6 +63,9 @@ public:
 #define UNKNOWN_MEMEFFECTS
 
 #include "swift/Runtime/RuntimeFunctions.def"
+
+      // swift_retainCount is not part of private contract between the compiler and runtime, but we still need to link it
+      linkEmbeddedRuntimeFunctionByName("swift_retainCount", { RefCounting });
   }
 
   void linkEmbeddedRuntimeFunctionByName(StringRef name,