Rollup merge of #101428 - JakobDegen:build-tests, r=oli-obk

Add mir building test directory

The first commit renames `mir-map.0` mir dumps to `built.after` dumps. I am happy to drop this commit if someone can explain the origin of the name.

The second commit moves a bunch of mir building tests into their own directory. I did my best to make sure that all of these tests are actually testing mir building, and not just incidentally using `built.after`

r? ``@oli-obk``

compiler/rustc_mir_transform/src/shim.rs

@@ -845,7 +845,7 @@ pub fn build_adt_ctor(tcx: TyCtxt<'_>, ctor_id: DefId) -> Body<'_> {
         span,
     );
 
-    rustc_middle::mir::dump_mir(tcx, None, "mir_map", &0, &body, |_, _| Ok(()));
+    crate::pass_manager::dump_mir_for_phase_change(tcx, &body);
 
     body
 }