Implement atomic floats

[lorentey]

Implement AtomicValue conformances for Float, Double and Float16. Requirements/constraints:

• compareExchange must compare the underlying encoded bit patterns rather than performing actual floating point comparisons. (I.e., it must consider negative zero different from positive zero, it must consider a NaN value equal to itself, etc.)
• We should provide as many direct operations as llvm/clang exposes. It would be nice to at least provide an atomic addition.
• We probably don't need/want an AtomicFloatingPoint protocol.