cmd/yacc: fix default action

cmd/yacc: fix default action

Previously, a production rule like

	A: B C D

would cause yacc to check that A and B have the same declared types,
but then it would generate an implicit action of { $$ = $3 } (i.e.,
copy the value from D), even if A and D have different types.

Fixes #10192.

Change-Id: I51cfd7baa0011557141dca33b7af1d892cc6f49e
Reviewed-on: https://go-review.googlesource.com/7780
Reviewed-by: Russ Cox <rsc@golang.org>