Add tracing in Miri for tcx.layout_of calls

[Stypox]

This PR is meant for Miri, but requires a few changes in rustc code, hence why it's here. It adds tracing capabilities to the layout_of function in tcx by overriding the layout_of query (under local_providers) with a wrapper that opens a tracing span and then calls the actual layout_of. To make this possible, I had to make rustc_ty_utils::layout::layout_of public. I added an assert to ensure the providers.layout_of value I am replacing is actually rustc_ty_utils::layout::layout_of, just in case.

I also considered taking the previous value in providers.layout_of and calling that one instead, to avoid making layout_of public. But then the closure would not be castable to a function pointer anymore (providers.layout_of is a function pointer), because it would depend on the local variable storing the previous value of providers.layout_of. Using a global variable would work but would rely on unsafe or on Mutexes, so I wanted to avoid it.

This PR also adds a tracing call in instantiate_from_frame_and_normalize_erasing_regions.

Here is some tracing output when Miri is run on src/tools/miri/tests/pass/hello.rs, visualizable in https://ui.perfetto.dev: trace-1750338860374637.json

Another place where I could have added tracing calls is to the rustc_middle::ty::layout::LayoutCx struct / spanned_layout_of() function, however there is no simple way to disable the tracing calls with compile-time boolean constants there (since LayoutCx::new() is used everywhere and referenced directly), and in any case it seems like spanned_layout_of() just calls tcx.layout_of() anyway. For completeness' sake, here is tracing output for when a tracing call is added to spanned_layout_of(): trace-1750340887920584.json

Note: this PR is not ready yet as it requires rust-lang/miri#4406 to disable the changes in the Miri code when "tracing" is disabled. I will also add more comments then, if you think my approach is ok.

r? @RalfJung