swift234

Eliminate the useless flag -enable-experimental-collection-casts.

This eliminates a pile of now-dead code in:
  * The type checker, where we no longer have special cases for bridging conversions
  * The expression ASTs, where we no longer need to distinguish bridging collection up/down casts
  * SILGen, which no longer uses

Still to come is the removal of the
_(set|dictionary)Bridge(From|To)ObjectiveC(Conditional)? entrypoints
from the standard library. They're still used by some tests.