constify vec![1, 2, 3] macro

[Lars-Schumann]

Tracking issues:
const_heap: #79597

makes all the parts needed for non-empty vec![] macros const:

alloc::boxed::box_assume_init_into_vec_unsafe
alloc::boxed::Box::assume_init
alloc::boxed::Box::into_raw_with_allocator
alloc::boxed::Box::new_uninit
alloc::slice::[T]::into_vec

Note that this does not allow for the use of the vec![(); 4] arm of this macro to be used in const-eval, since that uses specialization (spec and const traits don't really like each other so I didn't want to touch any of that in this).

[rustbot]

r? @TaKO8Ki

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Lars-Schumann]

r? @rust-lang/wg-const-eval