[perf] Delay the construction of early lint diag structs Attacks some of the perf regressions from https://github.com/rust-lang/rust/pull/124417#issuecomment-2123700666. 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.