[clang] Introduce paged vector (PR #66430)

[Giulio Eulisse via cfe-commits]

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@@ -501,7 +502,7 @@ class ASTReader
   ///
   /// When the pointer at index I is non-NULL, the declaration with ID
   /// = I + 1 has already been loaded.
-  std::vector<Decl *> DeclsLoaded;
+  llvm::PagedVector<Decl *> DeclsLoaded;
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ktf wrote:

I actually tried to change those as well, however the gain is less clear, at least in my simple ROOT tests. I guess one would need better tuning of PageSize for those. Part of the problem is that using a pointer per page is a bit heavy. I have some ideas on how to reduce this, by introducing another layer of indirection, but definitely not for this PR.

https://github.com/llvm/llvm-project/pull/66430