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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> 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.

I'm not sure premerge is the place for that. We want to be testing as standard of a configuration as possible, and since PCH is enabled by default for Flang, premerge should probably stick with that (barring other issues).

I hadn't seen #131137 until just now. That's useful context that I wish I had seen earlier...

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