[CSFix] Delay missing unwrap locator simplification until diagnostic

Instead of keeping two locators in the fix let's store only the
original locator and simplify it later in process of emitting
a diagnostic. That helps to avoid some duplicate work as well
as makes sure that locators supplied to the diagnostic always
have an anchor.

Resolves: rdar://problem/53344815

lib/Sema/CSDiagnostics.cpp

@@ -1085,6 +1085,14 @@ bool MissingOptionalUnwrapFailure::diagnoseAsError() {
 
   auto *anchor = getAnchor();
 
+  // If this is an unresolved member expr e.g. `.foo` its
+  // base type is going to be the same as result type minus
+  // r-value adjustment because base could be an l-value type.
+  // We want to fix both cases by only diagnose one of them,
+  // otherwise this is just going to result in a duplcate diagnostic.
+  if (getLocator()->isLastElement(ConstraintLocator::UnresolvedMember))
+    return false;
+
   if (auto assignExpr = dyn_cast<AssignExpr>(anchor))
     anchor = assignExpr->getSrc();