[PATCH] [libcxx] Fix __is_power2 and __next_power2. Change hashmap to handle new behavior.

[Eric Fiselier]

Ok, I'm abandoning this revision. I'm going to rename the functions so the names better suit the specialized functionality and implement different versions for PMR.
There is clearly a lot of interest in changes to the power of 2 / prime resizing of `__hash_table` but I think a better time to handle this all will be when I'm writing libc++ performance tests.
As Howard mentioned the performance of this code is critical and I don't want to make any changes without doing the proper performance testing legwork.

Thank's for all the input it has been invaluable.

http://reviews.llvm.org/D4948