Rollup merge of #144530 - Zalathar:instances-used, r=lqd

coverage: Infer `instances_used` from `pgo_func_name_var_map`

In obscure circumstances involving macro-expanded spans, we would sometimes emit a covfun record for a function with no physical coverage counters, and therefore no corresponding entry in the “PGO names” section of the binary. The absence of that name entry causes `llvm-cov` to fail with the cryptic error message:

```text
malformed instrumentation profile data: function name is empty
```

We can eliminate this mismatch by removing `instances_used` entirely, and instead inferring its contents from the keys of `pgo_func_name_var_map`.

This makes it impossible for a "used" function to lack a PGO name entry.

---

This is an attempt to eliminate the cause of rust-lang/rust#141577 when re-landing changes like rust-lang/rust#144298 in the future.

I haven't been able to reproduce the underlying issue in an in-tree test, because the only known repro involves a non-trivial derive proc-macro that relies on `syn` and `proc-macro2`. But I have manually verified in a separate branch that this change would have prevented the reoccurrence of https://github.com/rust-lang/rust/issues/141577#issuecomment-3120667286.

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