Auto merge of #83129 - LeSeulArtichaut:thir-unsafeck, r=nikomatsakis

Introduce the beginning of a THIR unsafety checker

This poses the foundations for the THIR unsafety checker, so that it can be implemented incrementally:
- implements a rudimentary `Visitor` for the THIR (which will definitely need some tweaking in the future)
- introduces a new `-Zthir-unsafeck` flag which tells the compiler to use THIR unsafeck instead of MIR unsafeck
- implements detection of unsafe functions
- adds revisions to the UI tests to test THIR unsafeck alongside MIR unsafeck

This uses a very simple query design, where bodies are unsafety-checked on a body per body basis. This however has some big flaws:
- the unsafety-checker builds the THIR itself, which means a lot of work is duplicated with MIR building constructing its own copy of the THIR
- unsafety-checking closures is currently completely wrong: closures should take into account the "safety context" in which they are created, here we are considering that closures are always a safe context

I had intended to fix these problems in follow-up PRs since they are always gated under the `-Zthir-unsafeck` flag (which is explicitely noted to be unsound).

r? `@nikomatsakis`
cc https://github.com/rust-lang/project-thir-unsafeck/issues/3 https://github.com/rust-lang/project-thir-unsafeck/issues/7

compiler/rustc_interface/src/tests.rs

@@ -736,6 +736,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(symbol_mangling_version, Some(SymbolManglingVersion::V0));
     tracked!(teach, true);
     tracked!(thinlto, Some(true));
+    tracked!(thir_unsafeck, true);
     tracked!(tune_cpu, Some(String::from("abc")));
     tracked!(tls_model, Some(TlsModel::GeneralDynamic));
     tracked!(trap_unreachable, Some(false));