[test][dsymutil] Fix tests for Windows bots.

The UNSUPPORTED directive was not honored by the bot, presumably because
of the FIXME above it. This moves the comment down and removes the
remaining update check from basic-linking-x86.test.

This should un-break: llvm-clang-x86_64-expensive-checks-win/builds/7798/

llvm-svn: 324598