Roll back r318260 because it is causing the windows bot to

break.  The alignas(__uint128_t) is not recognized with MSVC
it looks like.  Zachary, is there a similar type on windows?
I suppose I can go with alignas(16) here but I'd prefer to
specify the type alignment that I want & let the ABI dictate
how much padding is required.

llvm-svn: 318262