Rollup merge of #125816 - Zalathar:demangler, r=oli-obk

Don't build the `rust-demangler` binary for coverage tests

The coverage-run tests invoke `llvm-cov`, which requires us to specify a command-line demangler that it can use to demangle Rust symbol names.

Historically this used `src/tools/rust-demangler`, which means that we currently build two different command-line tools to help with the coverage tests (`rust-demangler` and `coverage-dump`).

However, it occurred to me that if we add a demangler mode to `coverage-dump` (which is only a handful of lines and no extra dependencies), then we only need to build one helper binary for the coverage tests, and there is no need for tests to build `rust-demangler` at all.

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Note that the `rust-demangler` binary is separate from the `rustc-demangle` crate (which both `rust-demangler` and `coverage-dump` use as a dependency to do the actual demangling).

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So the main benefits/motivations here are:
- Slightly faster builds after a fresh checkout or bootstrap bump.
- Making it clear that currently no tests actually need the `rust-demangler` binary, since the coverage tests can use their own tool instead.

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