go/types: add GCSizes

[josharian]

Provide a GCSizes that matches cmd/compile exactly and that all clients that depend on the same size computations as cmd/compile can use.

See #17584 for context.

[mdempsky]

What's the advantage of providing a separate GCSizes implementation, rather than just making StdSizes exactly match cmd/compile?

[griesemer]

@mdempsky That's an ok choice too, but I like to keep a version around that shows that there's no need to waste space in internal fragmentation (which is currently the case for gc-aligned structs used inside arrays or other structs).

[josharian]

If/when we look at this, we should make sure that GCSizes knows to add a padding byte at the end if the final struct field has size zero.

[gopherbot]

CL https://golang.org/cl/37666 mentions this issue.

[bradfitz]

Looks like this happened? What remains, @josharian?

[josharian]

Kind of. We now have a SizesFor that supports returning different concrete Sizes implementations, but we don't exactly have a concrete implementation for cmd/compile--it's close, but not exact, e.g. the padding byte in some circumstances. Moving to unplanned, though, pending direct need for it.

[cuonglm]

We have one now after #61035