swift234

Use AbstractionPattern to find Clang types for inputs and
results when finding bridged types, rather than reinventing
(well, pre-inventing) the same thing elsewhere.

Doing this apparently forces AbstractionPattern to deal
with ObjC protocol methods for the first time, which are
generic and therefore require even the Clang-based
abstraction patterns to propagate generic signatures.

Use this infrastructure to allow foreign error conventions
to suppress the wrapping of nonnull bridged collection results
in a level of optionality.  We can't treat nil results as
an empty collection while simultaneously treating them as
an error signal.

Swift SVN r28022