go/types: avoid "declared but not used errors" for invalid variable initializations

This is a partial port of CL 351669 from types2 to go/types; it
only copies the fix for variable usage.

Eventually we may want to use the compiler error messages for assignment
errors everywhere, but that doesn't need to happen now.

Change-Id: I62b024b1b29cc27c29d919de3de44f944f6e2b4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/351670
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/types/assignments.go

@@ -237,6 +237,7 @@ func (check *Checker) initVars(lhs []*Var, origRHS []ast.Expr, returnPos token.P
 	if len(lhs) != len(rhs) {
 		// invalidate lhs
 		for _, obj := range lhs {
+			obj.used = true // avoid declared but not used errors
 			if obj.typ == nil {
 				obj.typ = Typ[Invalid]
 			}