types: adjust validation_visitor construction for clang
types: adjust validation_visitor construction for clang

Clang does not implement P0960R3, parenthesized initialization of
aggregates, so we have to use brace initialization in
validation_visitor. It also does not implement class template
argument deduction for aggregates (P1816r0), so we have to
specify the template parameters explicity.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla