[ConstraintSystem] NFC: Add an accessor to determine whether type variable represents a key path

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -356,6 +356,10 @@ class TypeVariableType::Implementation {
   /// Determine whether this type variable represents a closure result type.
   bool isClosureResultType() const;
 
+  /// Determine whether this type variable represents
+  /// a type of a key path expression.
+  bool isKeyPathType() const;
+
   /// Retrieve the representative of the equivalence class to which this
   /// type variable belongs.
   ///

lib/Sema/TypeCheckConstraints.cpp

@@ -111,6 +111,10 @@ bool TypeVariableType::Implementation::isClosureResultType() const {
          locator->isLastElement<LocatorPathElt::ClosureResult>();
 }
 
+bool TypeVariableType::Implementation::isKeyPathType() const {
+  return locator && locator->isKeyPathType();
+}
+
 void *operator new(size_t bytes, ConstraintSystem& cs,
                    size_t alignment) {
   return cs.getAllocator().Allocate(bytes, alignment);