go/types, types2: fix SizesFor("gc", ...) to match actual gc behavior

[griesemer]

Replace https://go.dev/cl/501495 with proper fix in go/types.

[gopherbot]

Change https://go.dev/cl/506715 mentions this issue: go/types, types2: add Sizes that match actual gc behavior

[gopherbot]

Change https://go.dev/cl/506716 mentions this issue: cmd/compile: use types2.Sizes instead of compiler own implementation

[gopherbot]

Change https://go.dev/cl/506856 mentions this issue: go/types, types2: add Sizes that match actual gc behavior

[griesemer]

For future reference: The typechecker's StdSizes documentation explains how sizes are computed. If we want to use it for gc sizes, we need to either update the documentation for array and struct sizes, or add a flag to StdSizes that allows clients to choose the new behavior.

https://go.dev/cl/506856 already has an implementation that uses an internal (const) flag to select the appropriate sizes computation. We could simply move this flag into the StdSizes struct and expose it as a field. This way we don't change the existing behavior for backward-compatibility and also can enable the new behavior as needed.