Implement atomic floats

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.

Start by reviewing the existing AtomicValue conformances and the atomic floating-point operations exposed by llvm/clang. Implement conformances for Float, Double, and Float16, ensuring compareExchange uses encoded bit patterns and adding direct operations where supported. Done means the required behavior and operations are covered by tests.

Written by the indexing model from the issue text.