[CSSimplify] NFC: Perform assignments to a key path in a separate method

Previously key path type didn't have any requirements on their
root and value types but now that they do, we need a dedicated
place to perform an assignment and open/record all of the requirements.

lib/Sema/CSSimplify.cpp

@@ -4675,6 +4675,12 @@ ConstraintSystem::matchTypesBindTypeVar(
                : getTypeMatchFailure(locator);
   }
 
+  if (typeVar->getImpl().isKeyPathType()) {
+    return resolveKeyPath(typeVar, type, flags, locator)
+               ? getTypeMatchSuccess()
+               : getTypeMatchFailure(locator);
+  }
+
   assignFixedType(typeVar, type, /*updateState=*/true,
                   /*notifyInference=*/!flags.contains(TMF_BindingTypeVariable));
 
@@ -12257,6 +12263,15 @@ bool ConstraintSystem::resolveTapBody(TypeVariableType *typeVar,
   return !generateConstraints(tapExpr);
 }
 
+bool ConstraintSystem::resolveKeyPath(TypeVariableType *typeVar,
+                                      Type contextualType,
+                                      TypeMatchOptions flags,
+                                      ConstraintLocatorBuilder locator) {
+  assignFixedType(typeVar, contextualType, /*updateState=*/true,
+                  /*notifyInference=*/!flags.contains(TMF_BindingTypeVariable));
+  return true;
+}
+
 ConstraintSystem::SolutionKind
 ConstraintSystem::simplifyDynamicTypeOfConstraint(
                                         Type type1, Type type2,