Under certain circumstances involving shifts, go/types didn't verify
that untyped constant values were representable by the relevant type,
leading to the acceptance of incorrect programs (see the issue).
Fixing this code exposed another problem with int-to-string conversions
which suddenly failed because now the type-checker complained that a
(constant) integer argument wasn't representable as a string. Fixed that
as well.
Added many additional tests covering the various scenarious.
Found two cmd/compile bugs in the process (#21979, #21981) and filed
a go/types TODO (#21982).
Fixes #21727.
Change-Id: If443ee0230979cd7d45d2fc669e623648caa70da
Reviewed-on: https://go-review.googlesource.com/65370 Reviewed-by: Alan Donovan <adonovan@google.com>