The old code only worked for double-quoted strings, and only checked
that the end of the literal value was \n". This worked most of the time,
except for some strings like "foo\\n", which doesn't actually translate
into a trailing newline when unquoted.

To fix this, unquote the string first and look for a real newline at the
end of it. Ignore errors, as we don't have anything to do with string
literals using back quotes.

Fixes #22613.

Change-Id: I7cf96916dd578b7068216c2051ec2622cce0b740
Reviewed-on: https://go-review.googlesource.com/76194
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>