Check the reproducer from GitHub issue #1321, which uses a prefix label on a
begin-end block inside an `always_comb` process.
Check prefix labels on sequential and parallel blocks. Place attributes between
the labels and block keywords, verify that the sequential label creates a named
scope, cover all fork join types, and use matching closing labels.
Check separately that visible type identifiers can be shadowed by prefix labels
on sequential and parallel blocks.
Check that matching and different block names after `begin` or `fork` are
rejected when a prefix label is already present.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Lars-Peter Clausen