[clang] [NFC][clang][Serialization] Fix std::set_difference sorting mismatch in ASTReader (PR #219053)

[Owen Anderson via cfe-commits]

resistor wrote:

This error occurs in the LLVM 23.1.0 release (as I reported [here](https://github.com/llvm/llvm-project/issues/219046), so I would recommend this be cherry-picked.

https://github.com/llvm/llvm-project/pull/219053