[clang] [llvm] [Clang][AST][NFCI] Make Decl reference attributes directly (PR #219138)

[Erich Keane via cfe-commits]

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@@ -355,6 +351,13 @@ class alignas(8) Decl {
   LLVM_PREFERRED_TYPE(Linkage)
   mutable unsigned CacheValidAndLinkage : 3;
 
+  /// The attributes attached to this declaration, or null if it has none.
+  ///
+  /// This pointer is the sole record of whether the declaration has
+  /// attributes, so it must be cleared whenever the vector becomes empty.
+  /// Owned by the ASTContext that allocated it.
+  AttrVec *Attrs = nullptr;
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erichkeane wrote:

Another thing: @zygoloid mentioned on another thread that there were bits we should/could steal back from constructors.

I wonder if we could look through any of ours and do the reverse: switch it to a map.  I find myself wondering how often 'used/referenced' is actually meaningfully used.  I suspect it is JUST for a certain diagnostic, plus 1x in codegen.  Both of which are pretty rare lookups.  (compared to 'invalid', which gets looked up constantly).

`Implicit` might be something we could steal back, I think we only check that occasionally.  `TopLevelDeclInObjCContainer` I have no knowledge of, so we could very likely evaluate if that is looked up often enough to matter (OR is only used on a sub-type?).

All that to say, I think a static_ assert to make sure these types don't grow, plus the ability to 'steal' bits in the future if they are valuable should be sufficient? 

https://github.com/llvm/llvm-project/pull/219138