diagnostics: reject sanitizer compiles of proc-macro

[durin42]

We discussed this at the project all hands way back in May, and I'm only just now getting to it. Since proc-macros get loaded into rustc, it's not helpful to try and compile them in a sanitizer mode because you'll most likely end up with incompatible uses of things like the sanitizer runtimes.

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

I used an LLM to figure out where to plug this logic in, but I either hand-wrote or scrutinized every line before even committing it, and then self-reviewed again in the web UI before submitting the pull request.

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