[Diagnostics] If aggregate fix consists of identical fixes diagnose it specially

If aggregate fix (based on callee locator) differs only in picked
overload choices, let's diagnose that via `diagnoseForAmbiguity`
associated with a particular fix kind all solutions share, that
would produce a tailored diagnostic instead of the most general one.

Author: xedin

lib/Sema/CSFix.h

@@ -562,6 +562,10 @@ class ForceOptional final : public ContextualMismatch {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
   static ForceOptional *create(ConstraintSystem &cs, Type fromType, Type toType,
                                ConstraintLocator *locator);
 };
@@ -671,6 +675,10 @@ class GenericArgumentsMismatch final
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
   static GenericArgumentsMismatch *create(ConstraintSystem &cs, Type actual,
                                           Type required,
                                           llvm::ArrayRef<unsigned> mismatches,
@@ -1531,10 +1539,6 @@ class IgnoreContextualType : public ContextualMismatch {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
-  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
-    return diagnose(*commonFixes.front().first);
-  }
-
   static IgnoreContextualType *create(ConstraintSystem &cs, Type resultTy,
                                       Type specifiedTy,
                                       ConstraintLocator *locator);

lib/Sema/ConstraintSystem.cpp

@@ -3074,6 +3074,20 @@ static bool diagnoseAmbiguity(
 
   auto &DE = cs.getASTContext().Diags;
 
+  {
+    auto fixKind = aggregateFix.front().second->getKind();
+    if (llvm::all_of(
+            aggregateFix, [&](const std::pair<const Solution *,
+                                              const ConstraintFix *> &entry) {
+              auto &fix = entry.second;
+              return fix->getKind() == fixKind && fix->getLocator() == locator;
+            })) {
+      auto *primaryFix = aggregateFix.front().second;
+      if (primaryFix->diagnoseForAmbiguity(aggregateFix))
+        return true;
+    }
+  }
+
   auto *decl = ambiguity.choices.front().getDeclOrNull();
   if (!decl)
     return false;
@@ -3234,6 +3248,7 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
   for (const auto &entry : fixes) {
     const auto &solution = *entry.first;
     const auto *fix = entry.second;
+
     auto *calleeLocator = solution.getCalleeLocator(fix->getLocator());
 
     fixesByCallee[calleeLocator].push_back({&solution, fix});