[CSDiagnostics] Add a diagnostic for an attempt to specialize a concrete type

Diagnose attempts to specialize a concrete type or its alias:

```swift
struct Test {}
typealias X = Test

_ = X<Int>() // error
```

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -9162,3 +9162,8 @@ bool InvalidMemberReferenceWithinInitAccessor::diagnoseAsError() {
   emitDiagnostic(diag::init_accessor_invalid_member_ref, MemberName);
   return true;
 }
+
+bool ConcreteTypeSpecialization::diagnoseAsError() {
+  emitDiagnostic(diag::not_a_generic_type, ConcreteType);
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -3049,6 +3049,26 @@ class InvalidMemberReferenceWithinInitAccessor final
   bool diagnoseAsError() override;
 };
 
+/// Diagnose attempts to specialize a concrete type or its alias:
+///
+/// \code
+/// struct Test {}
+/// typealias X = Test
+///
+/// _ = X<Int>() // error
+/// \endcode
+class ConcreteTypeSpecialization final : public FailureDiagnostic {
+  Type ConcreteType;
+
+public:
+  ConcreteTypeSpecialization(const Solution &solution, Type concreteTy,
+                             ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator),
+        ConcreteType(resolveType(concreteTy)) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -2832,7 +2832,8 @@ AllowInvalidMemberReferenceInInitAccessor::create(ConstraintSystem &cs,
 
 bool AllowConcreteTypeSpecialization::diagnose(const Solution &solution,
                                                bool asNote) const {
-  return false;
+  ConcreteTypeSpecialization failure(solution, ConcreteType, getLocator());
+  return failure.diagnose(asNote);
 }
 
 AllowConcreteTypeSpecialization *