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

[Samarth Narang via cfe-commits]

snarang181 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.

OK. In any case, the usage of `llvm_any_of` seemed better than using an explicit `for-loop`. 

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