Previously, SILGen assumed that a foreign function could either have a
foreign async convention or a foreign error convention, but if it had
both, the error would be subsumed into the completion. That resulted in
failures to emit code for async calls of functions like
```
- (BOOL)minimalWithError:(NSError* _Nullable*)error
completionHandler:(void (^ _Nonnull)(void))completionHandler;
```
Here, SILGen gains the ability to emit such functions. To enable that,
a few changes were required when both conventions are present:
- a separate argument for each convention is used
- the ResultPlan is a ForeignErrorResultPlan nesting a
ForeignAsyncResultPlan
- the continuation is always of the form UnsafeContinuation<_, Error>
regardless of whether the completion handler takes an error
- the foreign error block fills the continuation with the error that was
passed by reference out of the ObjC method call
- the foreign error block branches to the block containing the await
instruction
rdar://80704984
