go/types: fewer spurious "declared but not used" follow-on errors

go/types: fewer spurious "declared but not used" follow-on errors

Mark variables as used even when they appear within an expression
context which we can't type-check; e.g., because the expression is
erroneous, or comes from an import "C" declaration.

Fixes #20358.

Change-Id: Ib28cc78d3867c597c7a1ace54de09ada02f5b33a
Reviewed-on: https://go-review.googlesource.com/43500
Reviewed-by: Alan Donovan <adonovan@google.com>