Revert "Try a third time to fix MSVC build after r234290"

This reverts commit r234295 (and r234294 and r234292 before it).  I
removed the implicit conversion to `MDTuple*` r234326, so there's no
longer an ambiguity in `operator[]()`.

I think MSVC should accept the original code now...

llvm-svn: 234335