(PUP-2357) Add validation of 'idem' (no-effect) expr sequences
(PUP-2357) Add validation of non-productive expressions

This adds validation of non-productive expressions:
* when they do not appear last
* when they appear last in constructs that do not produce a value
(maint) Remove empty unused method from test.
Merge pull request #2583 from hlindberg/pup-2357_validate-expr-no-effect

(PUP-2357) Validate non-productive expressions