include `HirId`s directly in the THIR, not wrapped in `LintLevel`s
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 `HirId`s aren't present in the THIR slightly simplifies diagnostics/lints/tools that want to map from the THIR back to the HIR, e.g. rust-lang/rust#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 rust-lang/rust#145569, since this was discussed there
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