swift234

Type-check catch patterns in closures as necessary in order
to decide whether they're exhaustive.

Unfortunately, we can't actually just type-check the pattern
because it might be dependent on other local type-checking state,
like the types of arguments or variables defined within the closure.
So instead we try to recognize a very specific pattern shape that
should be safe to coerce.

Also, this potentially introduces redundant diagnostics because
of the double-check, but it's tricky to do anything about it.
I think it's better to have these potential redundancies than
to infer a throwing closure type with 'catch let e as NSError',
though.

rdar://21715350

Swift SVN r30701