align_offset, align_to: no longer allow implementations to spuriously fail to align

[RalfJung]

For a long time, we have allowed align_offset to fail to compute a properly aligned offset, and align_to to return a smaller-than-maximal "middle slice". This was done to cover the implementation of align_offset in const-eval and Miri. See #62420 for more background. For about the same amount of time, this has caused confusion and surprise, where people didn't realize they have to write their code to be defensive against align_offset failures.

Another way to put this is: the specification is effectively non-deterministic, and non-determinism is hard to test for -- in particular if the implementation everyone uses to test always produces the same reliable result, and nobody expects it to be non-deterministic to begin with.

With #117840, Miri has stopped making use of this liberty in the spec; it now always behaves like rustc. That only leaves const-eval as potential motivation for this behavior. I do not think this is sufficient motivation. Currently, none of the relevant functions are stably const: align_offset is unstably const, align_to is not const at all. I propose that if we ever want to make these const-stable, we just accept the fact that they can behave differently at compile-time vs at run-time. This is not the end of the world, and it seems to be much less surprising to programmers than unexpected non-determinism. (Related: rust-lang/rfcs#3352.)

@thomcc has repeatedly made it clear that they strongly dislike the non-determinism in align_offset, so I expect they will support this. @oli-obk, what do you think? Also, whom else should we involve? The primary team responsible is clearly libs-api, so I will nominate this for them. However, allowing const-evaluated code to behave different from run-time code is t-lang territory. The thing is, this is not stabilizing anything t-lang-worthy immediately, but it still does make a decision we will be bound to: if we accept this change, then

• either align_offset/align_to can never be called in const fn,
• or we allow compile-time behavior to differ from run-time behavior.

So I will nominate for t-lang as well, with the question being: are you okay with accepting either of these outcomes (without committing to which one, just accepting that it has to be one of them)? This closes the door to "have align_offset and align_to at compile-time and also always have compile-time behavior match run-time behavior".

Closes #62420