[CSGen] Use a correct locator for subtype between explicit and inferred root types

Covariant conversions between explicit and contextual/inferred root
types of a key path are simulated through a subtype constraint which
has to be attached to a `KeyPathRoot` element.

Author: xedin

lib/Sema/CSGen.cpp

@@ -3742,9 +3742,10 @@ namespace {
         // Allow \Derived.property to be inferred as \Base.property to
         // simulate a sort of covariant conversion from
         // KeyPath<Derived, T> to KeyPath<Base, T>.
-        CS.addConstraint(ConstraintKind::Subtype, rootObjectTy, root, locator);
+        CS.addConstraint(ConstraintKind::Subtype, rootObjectTy, root,
+                         rootLocator);
       }
-      
+
       bool didOptionalChain = false;
       // We start optimistically from an lvalue base.
       Type base = LValueType::get(root);

test/Constraints/keypath.swift

@@ -81,8 +81,8 @@ func testVariadicKeypathAsFunc() {
 
   // These are not okay, the KeyPath should have a base that matches the
   // internal parameter type of the function, i.e (S...).
-  let _: (S...) -> Int = \S.i // expected-error {{key path value type 'S' cannot be converted to contextual type 'S...'}}
-  takesVariadicFnWithGenericRet(\S.i) // expected-error {{key path value type 'S' cannot be converted to contextual type 'S...'}}
+  let _: (S...) -> Int = \S.i // expected-error {{key path with root type 'S...' cannot be applied to a base of type 'S'}}
+  takesVariadicFnWithGenericRet(\S.i) // expected-error {{key path with root type 'S...' cannot be applied to a base of type 'S'}}
 }
 
 // rdar://problem/54322807

test/expr/unary/keypath/keypath.swift

@@ -692,8 +692,7 @@ func testSubtypeKeypathClass(_ keyPath: ReferenceWritableKeyPath<Base, Int>) {
 
 func testSubtypeKeypathProtocol(_ keyPath: ReferenceWritableKeyPath<PP, Int>) {
   testSubtypeKeypathProtocol(\Base.i)
-  // expected-error@-1 {{cannot convert value of type 'ReferenceWritableKeyPath<Base, Int>' to expected argument type 'ReferenceWritableKeyPath<any PP, Int>'}}
-  // expected-note@-2 {{arguments to generic parameter 'Root' ('Base' and 'any PP') are expected to be equal}}
+  // expected-error@-1 {{key path with root type 'any PP' cannot be applied to a base of type 'Base'}}
 }
 
 // rdar://problem/32057712