go/types: more robust behavior in the presence errors (due to import "C")

- Don't complain about invalid constant type if the type is
  invalid already (we do this in other places as well). This
  is useful to do in general, and even more so if we have
  invalid types due to import "C".

- Type-check the lhs of an assignment even if we bail out early
  due to an error on the rhs. This was simply an oversight. We
  already have machinery in place to "use" expressions; in this
  case we just have to also make sure we don't overcount "uses"
  of variables on the lhs.

- Fix overcount uses correction in assignments: Only do it if
  the variable in question is declared inside the same package
  to avoid possible race conditions when type-checking exported
  variables concurrently.

Fixes #22090.

Change-Id: I4c1b59f9ce38970e7129fedc5f6023908386e4f1
Reviewed-on: https://go-review.googlesource.com/88375
Reviewed-by: Alan Donovan <adonovan@google.com>

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