[ConstraintSystem] Add a way to check whether type variable represents a key path root type

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -491,6 +491,10 @@ class TypeVariableType::Implementation {
   /// a type of a key path expression.
   bool isKeyPathType() const;
 
+  /// Determine whether this type variable represents a root type of a key path
+  /// expression.
+  bool isKeyPathRoot() const;
+
   /// Determine whether this type variable represents a value type of a key path
   /// expression.
   bool isKeyPathValue() const;

lib/Sema/TypeCheckConstraints.cpp

@@ -144,6 +144,10 @@ bool TypeVariableType::Implementation::isKeyPathType() const {
   return locator && locator->isKeyPathType();
 }
 
+bool TypeVariableType::Implementation::isKeyPathRoot() const {
+  return locator && locator->isKeyPathRoot();
+}
+
 bool TypeVariableType::Implementation::isKeyPathValue() const {
   return locator && locator->isKeyPathValue();
 }