Remove the linked list through metadata caches, since LLDB is

no longer using it even in fallback code.

Author: rjmccall

stdlib/public/runtime/Metadata.cpp

@@ -157,7 +157,7 @@ namespace {
   struct GenericCacheEntry;
 
   // The cache entries in a generic cache are laid out like this:
-  struct GenericCacheEntryHeader : CacheEntry<GenericCacheEntry> {
+  struct GenericCacheEntryHeader {
     const Metadata *Value;
     size_t NumArguments;
   };

stdlib/public/runtime/MetadataCache.h

@@ -125,13 +125,7 @@ class KeyDataRef {
 };
 
 template <class Impl>
-struct CacheEntryHeader {
-  /// LLDB walks this list.
-  /// FIXME: when LLDB stops walking this list, there will stop being
-  /// any reason to store argument data in cache entries, and a *ton*
-  /// of weird stuff here will go away.
-  const Impl *Next;
-};
+struct CacheEntryHeader {};
 
 /// A CRTP class for defining entries in a metadata cache.
 template <class Impl, class Header = CacheEntryHeader<Impl> >
@@ -272,11 +266,6 @@ template <class ValueTy> class MetadataCache {
   static_assert(sizeof(Map) == 2 * sizeof(void*),
                 "offset of Head is not at proper offset");
 
-  /// The head of a linked list connecting all the metadata cache entries.
-  /// TODO: Remove this when LLDB is able to understand the final data
-  /// structure for the metadata cache.
-  const ValueTy *Head;
-
   struct ConcurrencyControl {
     Mutex Lock;
     ConditionVariable Queue;
@@ -362,10 +351,6 @@ template <class ValueTy> class MetadataCache {
     // creating the metadata.
     auto value = builder();
 
-    // Update the linked list.
-    value->Next = Head;
-    Head = value;
-
 #if SWIFT_DEBUG_RUNTIME
         printf("%s(%p): created %p\n",
                ValueTy::getName(), (void*) this, value);