[builtin] Remove "unsafeGuaranteed" and related code since Unmanaged now has an Ownership SSA based implementation that works completely in SILGen.

This isn't used in the stdlib anymore as well.

lib/SILOptimizer/Transforms/SpeculativeDevirtualizer.cpp

@@ -372,17 +372,6 @@ static bool tryToSpeculateTarget(FullApplySite AI, ClassHierarchyAnalysis *CHA,
                                  OptRemark::Emitter &ORE) {
   ClassMethodInst *CMI = cast<ClassMethodInst>(AI.getCallee());
 
-  // Don't devirtualize withUnsafeGuaranteed 'self' as this would prevent
-  // retain/release removal.
-  //   unmanged._withUnsafeGuaranteedRef { $0.method() }
-  if (auto *TupleExtract = dyn_cast<TupleExtractInst>(CMI->getOperand()))
-    if (auto *UnsafeGuaranteedSelf =
-            dyn_cast<BuiltinInst>(TupleExtract->getOperand()))
-      if (UnsafeGuaranteedSelf->getBuiltinKind() ==
-              BuiltinValueKind::UnsafeGuaranteed &&
-          TupleExtract->getFieldIndex() == 0)
-        return false;
-
   // Strip any upcasts off of our 'self' value, potentially leaving us
   // with a value whose type is closer (in the class hierarchy) to the
   // actual dynamic type.