include HirIds directly in the THIR, not wrapped in LintLevels

[dianne]

Occurrences of LintLevel in the THIR were always LintLevel::Explicit, containing a HirId, so we don't need to make it possible to put LintLevel::Inherited there. Removing the unused case where HirIds aren't present in the THIR slightly simplifies diagnostics/lints/tools that want to map from the THIR back to the HIR, e.g. #145569.

Since LintLevel is no longer present in the THIR, I've moved it in the second commit to live in rustc_mir_build; that's where it's actually used. I'm not sure exactly where exactly it should live there, but I put it in the builder::scope module since it's used by Builder::in_scope for determining when to introduce source scopes.

r? lcnr as the reviewer of #145569, since this was discussed there

[rustbot]

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred in match lowering

cc @Nadrieril