[clang] [llvm] remove redundant uses of `isa` caught by clang-tidy (NFC) (PR #192813)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

In general, you should fix every line you're changing that currently uses auto to use an explicit time in line with the general LLVM rules.

The naming of variables in this code seems depressingly random, but it's not something to address in this PR.

https://github.com/llvm/llvm-project/pull/192813