- use same local variable name (lno) for line number for LCOLAS everywhere
- remove now unneeded assignment of line number to yylval.i in lexer

Fix per suggestion of mdempsky.

Fixes #13415.

Change-Id: Ie3c7f5681615042a12b81b26724b3a5d8a979c25
Reviewed-on: https://go-review.googlesource.com/17248
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>