Rollup merge of #125483 - workingjubilee:move-transform-validate-to-mir-transform, r=oli-obk

compiler: validate.rs belongs next to what it validates

It's hard to find code that is deeply nested and far away from its callsites, so let's move `rustc_const_eval::transform::validate` into `rustc_mir_transform`, where all of its callers are. As `rustc_mir_transform` already depends on `rustc_const_eval`, the added visible dependency edge doesn't mean the dependency tree got any worse.

This also lets us unnest the `check_consts` module.

I did look into moving everything inside `rustc_const_eval::transform` into `rustc_mir_transform`. It turned out to be a much more complex operation, with more concerns and real edges into the `const_eval` crate, whereas this was both faster and more obvious.

Author: matthiaskrgr

tests/panic/mir-validation.stderr

@@ -1,4 +1,4 @@
-thread 'rustc' panicked at compiler/rustc_const_eval/src/transform/validate.rs:LL:CC:
+thread 'rustc' panicked at compiler/rustc_mir_transform/src/validate.rs:LL:CC:
 broken MIR in Item(DefId) (after phase change to runtime-optimized) at bb0[1]:
 (*(_2.0: *mut i32)), has deref at the wrong place
 stack backtrace: