go/types: use check.lookup consistently where possible (cleanup)

This CL converts the last call to scope.LookupParent with no position
information to a check.lookup call that respects position information
provided by Eval (there's one more LookupParent call that cannot be
converted, see the respective comment in the code).

In this case, the lookup is needed to determine the variable on the
LHS of an assignment, for adjustment of its `used` information.

Outside a types.Eval call, i.e., during normal type-checking, there
is no difference between this new code and the old code.

While in a types.Eval call, it's important to use the correct position
to look up the relevant variable. If token.NoPos were used, one might
find another variable with the same name, declared later in the scope.

Caveat: Types.Eval only accepts expressions, and it's currently not
possible to evaluate assignments (except via function literals, but
then the scope is different). That is, this change is a fix for a
potential future bug, and for now a no-op.

Change-Id: I28db1fe1202c07e3f7b3fadfd185728afb9b2ae7
Reviewed-on: https://go-review.googlesource.com/85199Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/assignments.go

@@ -153,7 +153,7 @@ func (check *Checker) assignVar(lhs ast.Expr, x *operand) Type {
 	var v *Var
 	var v_used bool
 	if ident != nil {
-		if _, obj := check.scope.LookupParent(ident.Name, token.NoPos); obj != nil {
+		if obj := check.lookup(ident.Name); obj != nil {
 			// It's ok to mark non-local variables, but ignore variables
 			// from other packages to avoid potential race conditions with
 			// dot-imported variables.