Auto merge of #136828 - yotamofek:pr/rustdoc/more-laziness, r=GuillaumeGomez,aDotInTheVoid

Do more lazy-formatting in `librustdoc` 🦥

Modify some formatting to be lazy, i.e. to not allocate interim strings that are later formatted into different strings.
Commits are small and stand on their own, and should mostly compile separately. (The first one doesn't compile due to `dead_code` because all it does is introduce a helper used in later commits)

Really excited about this one, local perf results are really good. I'd love a perf run to see how this looks on CI. This is the comparison of `instructions:u` count between master and this PR, on my computer:

# Summary
| | Range | Mean | Count |
|:---:|:---:|:---:|:---:|
| Regressions | - | 0.00% | 0 |
| Improvements | -8.03%, -0.40% | -2.93% | 5 |
| All | -8.03%, -0.40% | -2.93% | 5 |

# Primary benchmarks
| Benchmark | Profile | Scenario | % Change | Significance Factor |
|:---:|:---:|:---:|:---:|:---:|
| typenum-1.17.0 | doc | full | -8.03% | 40.16x |
| nalgebra-0.33.0 | doc | full | -4.19% | 20.97x |
| stm32f4-0.14.0 | doc | full | -1.35% | 6.73x |
| libc-0.2.124 | doc | full | -0.67% | 3.33x |
| cranelift-codegen-0.82.1 | doc | full | -0.40% | 1.99x |

Author: bors