utils: make basic UUID constructors constexpr
utils: make basic UUID constructors constexpr

Mark default and `UUID(most_sig_bits, least_sig_bits)` ctors
as constexpr.

This allows to construct constexpr constants using UUID type.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>
Message-Id: <20210429170630.533596-2-2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

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