Auto merge of #91352 - nnethercote:RawVec-reserve_for_push, r=dtolnay

Auto merge of #91352 - nnethercote:RawVec-reserve_for_push, r=dtolnay

Introduce `RawVec::reserve_for_push`.

If `Vec::push`'s capacity check fails it calls `RawVec::reserve`, which
then also does a capacity check.

This commit introduces `reserve_for_push` which skips the redundant
capacity check, for some slight compile time speed-ups.

I tried lots of minor variations on this, e.g. different inlining
attributes. This was the best one I could find.

r? `@ghost`