go/types, cmd/compile/internal/types2: avoid "declared but not used" errors for variables in invalid declarations

[griesemer]

Reminder/follow-up issue.

Some of these error have been addressed with 274615 in the dev.typeparams branch (types2 package only, so far), but not all. Should look into solving this more consistently.

For some test cases which could be shorter (no need for _ = ... statements), see:

test/fixedbugs/bug062.go
test/fixedbugs/bug131.go
test/fixedbugs/bug175.go
test/fixedbugs/bug289.go
test/interface/pointer.go

There are likely many more.

cc: @findleyr