There are parallel overloads of algorithms in <memory>, so it should define
the macro. Also, <execution> defines the exec policies for use with the
algos, so that should define the macro too.

Proposed resolution:

In [support.limits.general] add <memory> and <execution> to the table row
for __cpp_lib_parallel_algorithm.


| __cpp_lib_parallel_algorithm |
| 201603L |
| <algorithm> <INS><execution> <memory</INS> <numeric> |