[llvm] [cmake] Preserve CMake file API queries when clearing an LLVM external project build directory (PR #206801)

[Ian Anderson via llvm-commits]

ian-twilightcoder wrote:

> > `runtimes-clear` gets run automatically when the stamps are off, I don't know why
> 
> That sounds unintended and I have never observed this. Looking into the files, there is no dependency on `*-clear`, and the add_custom_target has no DEPENDS. I have no idea how it could run automatically other than that you configured your IDE to do so.

I can't actually reproduce it, and I based this PR on what Claude inferred had happened. Maybe this was an AI hallucination then. I'll just close the PR and open it again if I get definitive evidence it's needed.

https://github.com/llvm/llvm-project/pull/206801