Fix linker failures compiling Swift on Windows

[hughbe]

Fallout from #6460 where we changed from linking rpcrt4.lib to ole32.lib. I assumed that since ole32 included rpc APIs, they would work. I was wrong! Instead, this caused linker errors for undefined symbols for the following:

• UUIDToString is an undefined symbol, as we link ole32.lib instead of Rpcrt4.lib. Instead, use ole32.lib StringFromCLSID. Adjust for this API using wchar_t.
• UUIDFromString is an undefined symbol, for the same reason. Instead, use ole32's CLSIDFromString. Adjust for this API using wchar_t.
• UUIDFromNil is an undefined symbol, for the same reason. Instead, just create a zero-initialized Guid.
• UUIDCompare is an undefined symboll, for the same reason. Instead, just compare memory

[slavapestov]

@swift-ci Please smoke test