Bug: Pure virtual methods are allowed in non-virtual classes

[DaveMcEwan]

Methods declared like pure virtual function foo or pure virtual task foo are only allowed inside a virtual class (declared like virtual class foo) or an interface class (declared like interface class foo). IEEE1800-2017 doesn't phrase this particularly well and it isn't stated as a footnote to Annex A, but it looks clear enough from clause 8.21 (page 186).

This test shows the problem: https://chipsalliance.github.io/sv-tests-results/?v=sv_parser+uvm-req+class_member_test_5