[5.5] [SILGen] Handled transforming Bridged? -> Swift.

[nate-chandler]

5.5 Summary: Fix compiler crash for called-as-async ObjC methods whose completions take complex optional types that get unwrapped by Swift.

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Explanation: When an ObjC method is called-as-async from Swift, SILGen creates a completion handler to pass to the ObjC method. That completion handler is sometimes responsible for unwrapping Optionals that ObjC passes in. To do that unwrapping, a common utility is used. That common utility needs to know how to deal with various cases of Optional-ness of the source/bridged/ObjC type and the target/native/Swift type.

Previously, that utility knew how to deal with three of the four cases of Optional-ness: (1) Optional<Bridged> -> Optional<Native>, (2) Bridged -> Optional<Native>, (3) Bridged -> Native. It didn't, however, handle the other case: (4) Optional<Bridged> -> Native. Consequently, SILGen failed to generate code for calling-as-async methods like the following:

typedef NSString *Flavor NS_EXTENSIBLE_STRING_ENUM;
- (void)getIceCreamFlavorWithCompletionHandler:(void (^)(NSError * __nullable error, Flavor __nullable flavor))completionHandler;
- (void)badWithCompletionHandler:(void (^)(void (^ _Nullable)(void), NSError * _Nullable, BOOL))completionHandler  __attribute__((swift_async_error(zero_argument, 3)));

Here, handling for that fourth case is added. The number of Optional wrappings that the bridged type has that the native type does not is passed in to the utility. When the utility recurs, it increments the number as appropriate. Then, in the portions of the function where transformations are done, the values are unwrapped an appropriate number of times. Mostly that means force unwrapping N times before doing the transformation. In the case of types that conform to _ObjectiveCBridgeable, however, it means force unwrapping the value N-1 times after doing the transformation because _ObjectiveCBridgeable._unconditionallyBridgeFromObjectiveC performs one layer of unwrapping itself. As a side note, the reason why this hadn't been more of a problem is because many handlers use types which conform to ObjectiveCBridgeable and as such were already unwrapped.

Issue: rdar://80704382

Original PR: #38773

Testing: New regression tests, Swift CI

Reviewed by: Joe Groff

Risk: Low.

Scope: Limited to Swift import of ObjC methods as async functions.

[nate-chandler]

@swift-ci please test

[nate-chandler]

@swift-ci please nominate

[nate-chandler]

@swift-ci please test

[nate-chandler]

@swift-ci please test