UUID.uuidString should return an upper String like on Darwin

[pvieito]

Report for NSUUID: https://bugs.swift.org/browse/SR-865

[xwu]

Could you please write a more descriptive comment above as to what's being fixed? The link is to a bug that's already been fixed for several years.

This PR also needs a test that fails without the patch and passes with the patch. Also, the same change should be committed to the Darwin Foundation overlay.

[pvieito]

It seems that now Foundation builds against the system libuuid instead of the included version at swift-corelibs-foundation/uuid/.

As uuid_unparse is not guaranteed to be uppercase, in Linux (at least in Ubuntu 16.04) it returns a lowercase string. We should be using uuid_unparse_upper (_cf_uuid_unparse_upper) to ensure it returns an uppercase string.

In the Darwin overlay it is OK to use _cf_uuid_unparse as in Apple Darwin.uuid uuid_unparse already returns an uppercase string (although the maintainers should probably update it to maintain the same codebase in both implementations).

[compnerd]

I believe that it is almost guaranteed to be lowercase. The RFC 4122 expects the UUID to be lower case. This was something that I had to fix in the compiler as well for Windows as the UUID generation code in OLE also generated it lower case.

[compnerd]

CC: @millenomi

[compnerd]

@swift-ci please test

[pvieito]

@swift-ci please test

[compnerd]

@swift-ci please test