Remove all of the legacy home-grown atomic operations LLVM provided

except for CompareAndSwap. That is the only one still being used
anywhere now that statistics have been moved onto std::atomic.

Also, add a warning to the header that we shouldn't introduce more uses
of these old style atomics and instead should be using C++11's
std::atomic facilities.

Really hoping that we can hammer out the last couple of users here and
replace them with something more localized and/or principled, but
figured this was a pretty good start. =]

Note that this patch will need to be reverted if r271504 needs to be
reverted as that removes the last user of these. However, the biggest
risk for that patch was MSVC 2013 and at least one bot has already
passed where it would have failed there. I've tested MSVC 2015 using
their web interfaces and other platforms seem fine, so I'm optimistic.

Differential Revision: http://reviews.llvm.org/D20901

llvm-svn: 271540