get_blanket_impls is the slowest part of rustdoc, and the coverage pass
completely ignores blanket impls. This stops running it at all, and also
removes some unnecessary checks in `calculate_doc_coverage` that ignored
the impl anyway.
We don't currently measure --show-coverage in perf.rlo, but I tested
this locally on cargo and it brought the time down from 2.9 to 1.6
seconds.
During the document coverage reporting with
```bash
rustdoc something.rs -Z unstable-options --show-coverage
```
the coverage report also includes parts of the code that are marked
with `#[allow(missing_docs)]`, which outputs lower numbers in the
coverage report even though these parts should be ignored for the
calculation.
Co-authored-by: Joshua Nelson <joshua@yottadb.com>
