Tweak an _MSC_VER ifdef to use typename with clang in a unittest

In theory, Clang should figure out how to parse this correctly without
typename, but since this is the last TU that Clang falls back on in the
self-host, I'm going to compromise and check for __clang__.

And now Clang can self-host on -win32 without fallback!  The 'check' and
'check-clang' targets both pass.