[clang] Introduce paged vector (PR #66430)

[Vassil Vassilev 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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vgvassilev wrote:

@zygoloid, @ChuanqiXu9 should we follow this pattern and make `IdentifiersLoaded`, `SelectorsLoaded`, `MacrosLoaded` and `SubmodulesLoaded` `llvm::PagedVector`s, too? They could benefit from the same improvements in larger workflows. Maybe outside of this PR.

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