[Allocator] Fix r206256 which got the enabling case backwards on these

overloads. This doesn't matter *that* much yet, but it will in
a subsequent patch. I had tested the original pattern, but not my
attempt to pacify MSVC. This at least appears to work. Still fixing the
rest of the fallout in the final patch that uses these overloads, but it
will follow shortly.

llvm-svn: 206259