[llvm] [CI] Disable precompiled headers in monolithic-*.sh (PR #143369)

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

No failures that I've seen, it seems to be working well. I don't even necessarily think the buildbots need it, depends where we want to make the trade-off with the build time.
This change is just so that there's an extra check to make sure the PRs actually build without pch to not accidentally end up in a situation where the code only builds if pch is enabled.

https://github.com/llvm/llvm-project/pull/143369