Change the meaning of a UUID with all zeros for data.

Previously, depending on how you constructed a UUID from data or a
StringRef, an input value of all zeros was valid (e.g. setFromData)
or not (e.g. setFromOptionalData).  Since there was no way to tell
which interpretation to use, it was done somewhat inconsistently.
This standardizes the meaning of a UUID of all zeros to Not Valid,
and removes all the Optional methods and their uses, as well as the
static factories that supported them.

Differential Revision: https://reviews.llvm.org/D132191