Enable -Wimplicit-fallthrough option for clang

Enable -Wimplicit-fallthrough option for clang

On clang, -Wimplicit-fallthrough requires annotations with attributes,
but on gcc, -Wimplicit-fallthrough is the same as
-Wimplicit-fallthrough=3, which allows annotations with comments.  In
order to enforce consistent annotations with attributes on both
compilers, we test first for -Wimplicit-fallthrough=5, which will
succeed on gcc, and if that is not found we test for
-Wimplicit-fallthrough.

Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/76a8efcd-925a-4eaf-bdd1-d972cd1a32ff%40eisentraut.org