[lld-macho] Implement LC_UUID

Apple devtools use this to locate the dSYM files for a given
binary.

The UUID is computed based on an MD5 hash of the binary's contents. In order to
hash the contents, we must first write them, but LC_UUID itself must be part of
the written contents in order for all the offsets to be calculated correctly.
We resolve this circular paradox by first writing an LC_UUID with an all-zero
UUID, then updating the UUID with its real value later.

I'm not sure there's a good way to test that the value of the UUID is
"as expected", so I've just checked that it's present.

Reviewed By: #lld-macho, compnerd, smeenai

Differential Revision: https://reviews.llvm.org/D89418