Small fix: looks like a short variable declaration with a type switch
checks to make sure the variable used had valid shape (ONAME, OTYPE, or
ONONAME) and rejects everything else. Then a new variable is declared.
If the symbol contained in the declaration was a named OLITERAL (still a
valid identifier obviously) it would be rejected, even though a new
variable would have been declared.

Fix adds this case to the check.

Added a test case from issue12413.

Fixes #12413

Change-Id: I150dadafa8ee5612c867d58031027f2dca8c6ebc
Reviewed-on: https://go-review.googlesource.com/15760Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>