[llvm] Reland [Allocator] Keep bump pointer at a minimum alignment (PR #205240)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Add a `MinAlign` template parameter (default 8, sizeof(size_t) on 64-bit platforms) so that the common case `Alignment <= MinAlign` can skip realigning `CurPtr`.

This is more efficient _when `Allocate` is inlined and both arguments are compile-time constants_, right? That was not immediately obvious to me from the patch.

https://github.com/llvm/llvm-project/pull/205240