[clang] [clang][NFC] Refactor replaceExternalDecls to use llvm::any_of (PR #143275)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

IIUC the `FIXME` comment means that the time complexity of this part of code is not good enough and we can make it faster.

https://github.com/llvm/llvm-project/pull/143275