cmd/compile: don't panic if unsafe.Sizeof/Offsetof is used with oversize types

cmd/compile: don't panic if unsafe.Sizeof/Offsetof is used with oversize types

In the Sizes API, recognize an overflow (to a negative value) as a
consequence of an oversize value, and specify as such in the API.

Adjust the various size computations to take overflow into account.

Recognize a negative size or offset as an error and report it rather
than panicking.

Use the same protocol for results provided by the default (StdSizes)
and external Sizes implementations.

Add a new error code TypeTooLarge for the new errors.

Fixes #59190.
Fixes #59207.

Change-Id: I8c33a9e69932760275100112dde627289ac7695b
Reviewed-on: https://go-review.googlesource.com/c/go/+/478919
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>