SL-19110 revert LLUUID::combine() to old algorithm to match server code. (#75)

As it happens, the change in the LLUUID::combine() algorithm introduced by one
of my previous commits is causing invalid assets creation (seen with
some clothing items, such as Shape and Universal types); obviously, the server
is using the old algorithm for UUID validation purpose of these assets.

This commit reverts LLUUID::combine() code to use LLMD5.