[clang] [clang][codegen] Add a verifier IR pass before any further passes. (PR #68015)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

The latest revision now has the new pass under control of the same flag.

I will drop the matter of whether we do it for release builds by default, for this MR at least, because otherwise this is a useful change.

https://github.com/llvm/llvm-project/pull/68015