[ConstraintSystem] Add a helper to determine whether locator is for argument to C/ObjC imported decl

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -5214,6 +5214,10 @@ class ConstraintSystem {
   /// as the anchor, and a path to the argument at index `0`.
   ConstraintLocator *getArgumentLocator(Expr *expr);
 
+  /// Determine whether given locator represents an argument to declaration
+  /// imported from C/ObjectiveC.
+  bool isArgumentOfImportedDecl(ConstraintLocatorBuilder locator);
+
   SWIFT_DEBUG_DUMP;
   SWIFT_DEBUG_DUMPER(dump(Expr *));
 

lib/Sema/ConstraintSystem.cpp

@@ -4999,6 +4999,22 @@ bool constraints::isArgumentOfReferenceEqualityOperator(
          isOperatorArgument(locator, "!==");
 }
 
+bool ConstraintSystem::isArgumentOfImportedDecl(
+    ConstraintLocatorBuilder locator) {
+  auto last = locator.last();
+  if (!(last && last->is<LocatorPathElt::ApplyArgToParam>()))
+    return false;
+
+  auto *application = getCalleeLocator(getConstraintLocator(locator));
+
+  auto overload = findSelectedOverloadFor(application);
+  if (!(overload && overload->choice.isDecl()))
+    return false;
+
+  auto *choice = overload->choice.getDecl();
+  return choice->hasClangNode();
+}
+
 ConversionEphemeralness
 ConstraintSystem::isConversionEphemeral(ConversionRestrictionKind conversion,
                                         ConstraintLocatorBuilder locator) {