Optimize documentation lints **a lot** (2/2) (7.5% -> 1%) (#14870)

So, after https://github.com/rust-lang/rust-clippy/pull/14693 was
merged,
this is the continuation. It performs some optimizations on
`Fragments::span`
, makes it so we don't call it so much, and makes a 85.75% decrease
(7.51% -> 1.07%)
in execution samples of `source_span_for_markdown_range` and a 6.39% ->
0.88%
for `core::StrSearcher::new`. Overall a 13.11% icount decrase on
docs-heavy crates.

Benchmarked mainly on `regex-1.10.5`.

@rustbot label +performance-project

This means that currently our heaviest function is
`rustc_middle::Interners::intern_ty`, even
for documentation-heavy crates

Along with rust-lang/rust-clippy#14693, this makes the lint a 7% of what
it was before and makes it so that even in the most doc-heavy of crates
it's not an issue.

changelog:Optimize documentation lints by a further 85%

r? @Jarcho

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