swift234

Restore operator '+' family for UnsafePointer. (#3719)

The reverts part of my previous patch. Removing the operators is too much of a
performance penalty to take. The difference is that the Strideable operators are
not transparent.

I still need to remove the UnsafeRawPointer operators, so -Onone performance
will be bad in some cases until this is fixed:
<rdar://problem/27513184> [perf] Strideable operators are not transparent. This is a huge -Onone performance penalty.