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

[Erich Keane via llvm-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:

Well... huh.  I don't see any of the 'well we can just steal a bit from here' areas to this either.  So this is something that we didn't realize happened at one point (probably OMP adding to the IdentifierNamespace).  

I still want to hear @AaronBallman 's feedback on this one, but I'm leaning towards in favor unless there is something I'm missing.

ONE thing as a 'lets improve things while we are here': Can we add a static-assert on `sizeof(Decl)` to make sure we don't accidentally grow it in the future? 

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