Hardcode typeof_unqual to __typeof_unqual__ for clang

Hardcode typeof_unqual to __typeof_unqual__ for clang

A new attempt was made in 63275ce84d2 to make typeof_unqual work on all
configurations of CC and CLANG. This re-introduced an old problem
though, where CLANG would only support __typeof_unqual__ but the
configure check for CC detected support for typeof_unqual.

This fixes that by always defining typeof_unqual as __typeof_unqual__
under clang.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/92f9750f-c7f6-42d8-9a4a-85a3cbe808f3%40eisentraut.org