[CS] Improve mutability diag for dynamic lookup

Look at the callee locator to add some additional
info to the diagnostic.

Author: hamishknight

lib/Sema/CSDiagnostics.cpp

@@ -1614,7 +1614,8 @@ bool RValueTreatedAsLValueFailure::diagnoseAsError() {
       }
     }
 
-    if (auto resolvedOverload = getOverloadChoiceIfAvailable(getLocator())) {
+    if (auto resolvedOverload =
+            getCalleeOverloadChoiceIfAvailable(getLocator())) {
       if (resolvedOverload->choice.getKind() ==
           OverloadChoiceKind::DynamicMemberLookup)
         subElementDiagID = diag::assignment_dynamic_property_has_immutable_base;

test/attr/attr_dynamic_member_lookup.swift

@@ -36,10 +36,10 @@ struct NonMutSettable {
 
 func test(a: Gettable, b: Settable, c: MutGettable, d: NonMutSettable) {
   global = a.wyverns
-  a.flavor = global // expected-error {{cannot assign to property: 'a' is a 'let' constant}}
+  a.flavor = global // expected-error {{cannot assign through dynamic lookup property: 'a' is a 'let' constant}}
 
   global = b.flavor
-  b.universal = global // expected-error {{cannot assign to property: 'b' is a 'let' constant}}
+  b.universal = global // expected-error {{cannot assign through dynamic lookup property: 'b' is a 'let' constant}}
   b.thing += 1 // expected-error {{left side of mutating operator isn't mutable: 'b' is a 'let' constant}}
 
   var bm = b
@@ -318,7 +318,7 @@ class DerivedClassWithSetter : BaseClass {
 func testOverrideSubscript(a: BaseClass, b: DerivedClassWithSetter) {
   let x = a.frotz + b.garbalaz
   b.baranozo = x
-  a.balboza = 12 // expected-error {{cannot assign to property}}
+  a.balboza = 12 // expected-error {{cannot assign through dynamic lookup property: 'a' is a 'let' constant}}
 }
 
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