Auto merge of #125410 - fmease:adj-lint-diag-api, r=nnethercote

[perf] Delay the construction of early lint diag structs

Attacks some of the perf regressions from rust-lang/rust#124417 (comment).

See individual commits for details. The first three commits are not strictly necessary.
However, the 2nd one (06bc4fc67145e3a7be9b5a2cf2b5968cef36e587, *Remove `LintDiagnostic::msg`*) makes the main change way nicer to implement.
It's also pretty sweet on its own if I may say so myself.

Author: bors