This is a completely refactored version of the core swift_dynamicCast
runtime method.

This fixes a number of bugs, especially in the handling of multiply-wrapped
types such as Optional within Any.  The result should be much closer to the
behavior specified by `docs/DynamicCasting.md`.

Most of the type-specific logic is simply copied over from the
earlier implementation, but the overall structure has been changed
to be uniformly recursive.  In particular, this provides uniform
handling of Optional, existentials, Any and other common "box"
types along all paths.  The consistent structure should also be
easier to update in the future with new general types.

Benchmarking does not show any noticable performance implications.

**Temporarily**, the old implementation is still available.  Setting the
environment variable `SWIFT_OLD_DYNAMIC_CAST_RUNTIME` before launching a program
will use the old runtime implementation.  This is only to facilitate testing;
once the new implementation is stable, I expect to completely remove the old
implementation.

The old `tryCastFromNil` could never fail (hence doesn't need to "try") and
doesn't need all the arguments we were giving it.

We know this memoization makes a big difference for String,
but the wins for Dictionary are less clear.  It's easy to
add it back later if it proves helpful.

Clarify the unreachable messaging a bit:
* "Unexpected" means I knew about it but thought it could
  never appear here
* "Unknown" means I didn't know about it when I wrote this code

…n-optimized builds