go/types, types2: fix range clause checks for constant range expressions

go/types, types2: fix range clause checks for constant range expressions

Add missing checks for the case where the range expression is
a (possibly untyped) constant integer expression.

Add context parameter to assignVar for better error message
where the expression is part of a range clause.

Also, rename s/expr/Expr/ where it denotes an AST expression,
for clarity.

Fixes #65133.
For #65137.

Change-Id: I72962d76741abe79f613e251f7b060e99261d3ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/556398
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>