In many places, there was an #ifdef/#else block that selected one of two implmentatio...

In many places, there was an #ifdef/#else block that selected one of two implmentations of rebind_alloc based on whether or not we had template aliases. Create a helper struct to encapsulate that bit of logic, and replace all the ifdefs with uses of that struct. No functionality change intented.

llvm-svn: 234296