go/types, types2: consistently set x.mode in Checker.assignment

go/types, types2: consistently set x.mode in Checker.assignment

Per the doc string, Checker.assignment must set x.mode to invalid
in case of failure.

(It may be simpler to return a bool, but the operand x may be tested
by callers several stack frames above.)

Change-Id: Ia1789b0396e8338103c0e707761c46f8d253fd31
Reviewed-on: https://go-review.googlesource.com/c/go/+/485875
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>