Add UUID::SetFromOptionalStringRef, use it in DynamicLoaderDarwin

We use UUID::fromOptionalData to read UUID's from the Mach-O files, so UUID's
of all 0's are invalid UUID's.
We also get uuid's from debugserver, which need to match the file UUID's.  So
we need an API that treats "000000000" as invalid as well.  Added that and use it.

Differential Revision: https://reviews.llvm.org/D57195

llvm-svn: 352122