[CSFix] Move diagnoseForAmbiguity to RequirementFix

The logic is common for all types of fixes that represent
a requirement failure.

(cherry picked from commit 9b688e9)

Author: xedin

include/swift/Sema/CSFix.h

@@ -602,6 +602,8 @@ class RequirementFix : public ConstraintFix {
   Type lhsType() const { return LHS; }
   Type rhsType() const { return RHS; }
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override;
+
   bool diagnose(const Solution &solution,
                 bool asNote = false) const override = 0;
 };
@@ -625,8 +627,6 @@ class MissingConformance final : public RequirementFix {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
-  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override;
-
   static MissingConformance *forRequirement(ConstraintSystem &cs, Type type,
                                             Type protocolType,
                                             ConstraintLocator *locator);

lib/Sema/CSFix.cpp

@@ -410,16 +410,16 @@ bool MissingConformance::diagnose(const Solution &solution, bool asNote) const {
   return failure.diagnose(asNote);
 }
 
-bool MissingConformance::diagnoseForAmbiguity(
+bool RequirementFix::diagnoseForAmbiguity(
     CommonFixesArray commonFixes) const {
-  auto *primaryFix = commonFixes.front().second->getAs<MissingConformance>();
+  auto *primaryFix = commonFixes.front().second;
   assert(primaryFix);
 
   if (llvm::all_of(
           commonFixes,
           [&primaryFix](
               const std::pair<const Solution *, const ConstraintFix *> &entry) {
-            return primaryFix->isEqual(entry.second);
+            return primaryFix->getLocator() == entry.second->getLocator();
           }))
     return diagnose(*commonFixes.front().first);