AST: make RequirementMachine/PropertyUnification.cpp build on VS2022

Using the qualified name is subtly different from scoping the function.
Use the latter, which correctly places the function into the namespace,
to enable building with VS2022.

Author: compnerd

lib/AST/RequirementMachine/PropertyUnification.cpp

@@ -650,11 +650,13 @@ RewriteSystem::TypeWitness::TypeWitness(
          getConcreteConformance().getProtocol());
 }
 
-bool swift::rewriting::operator==(
-    const RewriteSystem::TypeWitness &lhs,
-    const RewriteSystem::TypeWitness &rhs) {
-  return (lhs.LHS == rhs.LHS &&
-          lhs.RHS == rhs.RHS);
+namespace swift {
+namespace rewriting {
+bool operator==(const RewriteSystem::TypeWitness &lhs,
+                const RewriteSystem::TypeWitness &rhs) {
+  return lhs.LHS == rhs.LHS && lhs.RHS == rhs.RHS;
+}
+}
 }
 
 void RewriteSystem::TypeWitness::dump(llvm::raw_ostream &out) const {
@@ -906,4 +908,4 @@ void PropertyMap::recordConcreteConformanceRule(
   path.invert();
 
   inducedRules.emplace_back(std::move(lhs), std::move(rhs), std::move(path));
-}
+}