Rework FixedSizeTemplateParameterListStorage

[hubert.reinterpretcast]

Summary

This change replaces the custom FixedSizeTemplateParameterListStorage implementation with one that follows the interface provided by llvm::TrailingObjects.

[hubert.reinterpretcast]

Adjust for changes to D19770

[hubert.reinterpretcast]

This is superseded by D19322.

include/clang/AST/DeclTemplate.h

public:
   }
 
   friend TrailingObjects;
   template <size_t N> friend class FixedSizeTemplateParameterListStorage;
 };
 
 /// \brief Stores a list of template parameters for a TemplateDecl and its
 /// derived classes. Suitable for creating on the stack.
-template <size_t N> class FixedSizeTemplateParameterListStorage {
-  // This is kinda ugly: TemplateParameterList usually gets allocated
-  // in a block of memory with NamedDecls appended to it. Here, to get
-  // it stack allocated, we include the params as a separate
-  // variable. After allocation, the TemplateParameterList object
-  // treats them as part of itself.
-  TemplateParameterList List;
-  NamedDecl *Params[N];
+template <size_t N>
+class FixedSizeTemplateParameterListStorage
+    : public TemplateParameterList::FixedSizeStorageOwner {
+  typename TemplateParameterList::FixedSizeStorage<
+      NamedDecl *>::with_counts<
+      N>::type storage;
 
 public:
   FixedSizeTemplateParameterListStorage(SourceLocation TemplateLoc,
                                         SourceLocation LAngleLoc,
                                         ArrayRef<NamedDecl *> Params,
                                         SourceLocation RAngleLoc)
-      : List(TemplateLoc, LAngleLoc, Params, RAngleLoc) {
-    // Because we're doing an evil layout hack above, have some
-    // asserts, just to double-check everything is laid out like
-    // expected.
-    assert(sizeof(*this) ==
-               TemplateParameterList::totalSizeToAlloc<NamedDecl *>(N) &&
-           "Object layout not as expected");
-    assert(this->Params == List.getTrailingObjects<NamedDecl *>() &&
-           "Object layout not as expected");
-  }
-  TemplateParameterList *get() { return &List; }
+      : FixedSizeStorageOwner(new (static_cast<void *>(&storage))
+            TemplateParameterList(TemplateLoc, LAngleLoc, Params, RAngleLoc)) {}
 };
 
 /// \brief A template argument list.
 class TemplateArgumentList final
     : private llvm::TrailingObjects<TemplateArgumentList, TemplateArgument> {
   /// \brief The template argument list.
   const TemplateArgument *Arguments;