Currently the parts of session initialization that happen within
`rustc_interface` are split between `run_compiler` and `create_session`.
This split isn't necessary and obscures what's happening.

This commit merges the two functions. I think a single longer function
is much clearer than splitting this code across two functions in
different modules, especially when `create_session` has 13 parameters,
and is misnamed (it also creates the codegen backend). The net result is
43 fewer lines of code.

Clean up unchecked_math, separate out unchecked_shifts

Tracking issue: rust-lang#85122

Changes:

1. Remove `const_inherent_unchecked_arith` flag and make const-stability flags the same as the method feature flags. Given the number of other unsafe const fns already stabilised, it makes sense to just stabilise these in const context when they're stabilised.
2. Move `unchecked_shl` and `unchecked_shr` into a separate `unchecked_shifts` flag, since the semantics for them are unclear and they'll likely be stabilised separately as a result.
3. Add an `unchecked_neg` method exclusively to signed integers, under the `unchecked_neg` flag. This is because it's a new API and probably needs some time to marinate before it's stabilised, and while it *would* make sense to have a similar version for unsigned integers since `checked_neg` also exists for those there is absolutely no case where that would be a good idea, IMQHO.

The longer-term goal here is to prepare the `unchecked_math` methods for an FCP and stabilisation since they've existed for a while, their semantics are clear, and people seem in favour of stabilising them.

…h-on-panic, r=TaKO8Ki

Don't emit delayed good-path bugs on panic

This should fix rust-lang#117381, cc ``@RalfJung``

As opposed to delayed bugs, delayed *good path* bugs really don't make sense to show on panics.

…ck-suggestions, r=compiler-errors

Refactor: move suggestion functions from demand to suggestions

follow-up from
rust-lang#116841 (comment)

…li-obk

Inline and remove `create_session`.

Currently the parts of session initialization that happen within `rustc_interface` are split between `run_compiler` and `create_session`. This split isn't necessary and obscures what's happening.

This commit merges the two functions. I think a single longer function is much clearer than splitting this code across two functions in different modules, especially when `create_session` has 13 parameters, and is misnamed (it also creates the codegen backend). The net result is 43 fewer lines of code.

r? ``@oli-obk``