Still working on the basic design of <atomic>. I'm working towards a system...

Still working on the basic design of <atomic>.  I'm working towards a system by which the compiler only needs to define the strongest intrinsics it can.  Weaker atomics in the library automatically try stronger and stronger variants, picking the weakest compiler intrinsic available.  If no compiler intrinsics are available for a given operation, the library locks a mutex and does the job.  Better documentation to follow...

llvm-svn: 115538