[dev.typeparams] go/constant: in ToFloat, convert to rational numbers, not floats

Floating-point constants are represented as rational numbers when
possible (i.e., when numerators and denominators are not too large).
If we convert to floats when not necessary, we risk losing precision.

This is the minimal fix for the specific issue, but it's too aggressive:
If the numbers are too large, we still want to convert to floats.
Will address in a separate CL that also does a few related cleanups.

Fixes #43908.

Change-Id: Id575e34fa18361a347c43701cfb4dd7221997f66
Reviewed-on: https://go-review.googlesource.com/c/go/+/286552


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>