[Allocator] MSVC apparantly has broken SFINAE context handling of
'sizeof(T)' for T == void and produces a hard error. I cannot fathom why
this is OK. Oh well. switch to an explicit test for being the
(potentially qualified) void type, which is the only specific case I was
worried about. Hopefully this survives the libstdc++ build bots which
have limited type traits implementations...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206256 91177308-0d34-0410-b5e6-96231b3b80d8