The `Value.eql` function has to test for value equality *as-if* the lhs
value parameter is coerced into the type of the rhs. For tagged unions,
there was a problematic case when the lhs was an anonymous struct,
because in such case the value is empty_struct_value and the type
contains all the value information. But the only type available in the
function was the rhs type.

So the fix involved making `Value.eqlAdvanced` also accept the lhs type,
and then enhancing the logic to handle the case of the `.anon_struct` tag.

closes #12418

Tests run locally:
 * test-behavior
 * test-cases