[Allocator] Hoist the external helper function into a namespace scope
declaration. GCC 4.7 appears to get hopelessly confused by declaring
this function within a member function of a class template. Go figure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206152 91177308-0d34-0410-b5e6-96231b3b80d8