Auto merge of #117197 - Zalathar:demangler, r=onur-ozkan

Avoid unnecessary builds/rebuilds of `rust-demangler`

This is a combination of two loosely-related changes:

- Don't build `rust-demangler` as a dependency of `tests/run-make`, because after #112300 none of the remaining run-make tests actually use it. (If future run-make tests ever do need the demangler, it'll be easy to add it back.)
- For `tests/run-coverage`, build the demangler with the stage 0 compiler instead of the current-stage compiler. This avoids having to uselessly rebuild the demangler after modifying and rebuilding the compiler itself.

Author: bors