UUID: Add support for arbitrary-sized module IDs

Summary:
The data structure is optimized for the case where the UUID size is <=
20 bytes (standard length emitted by the GNU linkers), but larger sizes
are also possible.

I've modified the string conversion function to support the new sizes as
well. For standard UUIDs it maintains the traditional formatting
(4-2-2-2-6). If a UUID is shorter, we just cut this sequence short, and
for longer UUIDs it will just repeat the last 6-byte block as long as
necessary.

I've also modified ObjectFileELF to take advantage of the new UUIDs and
avoid manually padding the UUID to 16 bytes. While there, I also made
sure the computed UUID does not depend on host endianness.

Reviewers: clayborg, lemo, sas, davide, espindola

Subscribers: emaste, arichardson, lldb-commits

Differential Revision: https://reviews.llvm.org/D48633

llvm-svn: 335963