swift234

Rework the signature for ASTContext::getForeignRepresentable.

- Extract the cache entry type to a top-level type that can be used in the
  signature of the function.
- Rewrite the implementation of the type in terms of standard LLVM ADTs
  instead of manual bit manipulation.
- Add support for trivial-with-optional types (none of which exist yet).
- Drop the use of SIL/BridgedTypes.def; the only non-trivial non-object
  bridged type today is 'Bool' (for ObjCBool and DarwinBoolean) and that's
  already trivially bridged to CBool too.
- Save an Identifier lookup when possible.
- Finally, rename the function to getForeignRepresentationInfo.

No intended functionality change.