Always print UUIDs using uppercase.

[ezephir]

This changes UUID::toString() to always print using upper case. The previous
behavior was platform specific, resulting in difficulty checking UUIDs in tests.
Serialization/basic_sil.swift and SIL/Parser/basic.sil are now expected to pass
on Linux. This resolves bug SR-417.

[ezephir]

Additional notes for the review:

• The case that libuuid's unparse function uses seems to be defined at compile time.
• On Ubuntu 14.04, this is compiled as lower case. In OS X 10.10, this is compiled in as upper case.
• Parsing is unaffected by this change (upper, lower, and mixed case UUIDs still parse)

This change causes other platforms to conform with Swift's current behavior on OS X.

[lattner]

Nice!