[clang] [Clang] Reduce the size of Decl and classes derived from it (PR #87361)

[Erich Keane via cfe-commits]

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@@ -268,17 +268,34 @@ class alignas(8) Decl {
   ///   }
   ///   void A::f(); // SemanticDC == namespace 'A'
   ///                // LexicalDC == global namespace
-  llvm::PointerUnion<DeclContext*, MultipleDC*> DeclCtx;
+  llvm::PointerIntPair<
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erichkeane wrote:

I don't think it ends up being 'manual bit twiddling' so much as 3 values in an enum, and I also found the `getPointer().getPointer()` pretty mind-bending, but at least it is somewhat well hidden in stuff we won't touch for a while, so I won't insist.

https://github.com/llvm/llvm-project/pull/87361