interpret: make isize::MAX the limit for dynamic value sizes

[RalfJung]

We are currently enforcing data_layout.obj_size_bound() as the maximal dynamic size of a Rust value (including for size_of_val_raw), but that does not match the docs.

In particular, Miri currently falsely says that this code has UB:

#![feature(layout_for_ptr)]
fn main() {
    let size = isize::MAX as usize;
    // Creating a raw slice of size isize::MAX and asking for its size is okay.
    let s = std::ptr::slice_from_raw_parts(1usize as *const u8, size);
    assert_eq!(size, unsafe { std::mem::size_of_val_raw(s) });
}

[rust-highfive]

Some changes occured to the CTFE / Miri engine

cc @rust-lang/miri

Some changes occured to the CTFE / Miri engine

cc @rust-lang/miri

[rust-highfive]

r? @matthewjasper

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[RalfJung]

r? @oli-obk

[oli-obk]

@bors r+

[bors]

📌 Commit cb0d15b has been approved by oli-obk