[SIL] Fixed return index for call-as-async (BOOL, Error, Value) case.

When converting an ObjC method type which is being called as async to a
Swift function type, some of the values passed to the ObjC method's
completion handler are converted to return values of the Swift function.
The flag and error parameters, however, if present, are ignored.

When abstracting the result type for the Swift method, the formal type
of the corresponding parameter in the ObjC method's completion handler
is used.  Digging out that parameter entails indexing into the
parameters of the completion handler.  Previously, the indexing logic
relied on the error appearing before the flag if both appeared before
the value of interest.  Here, the indexing is tweaked to check both
special indices for each possible index until the first that matches
neither is found.

rdar://81625544

Author: nate-chandler

lib/SIL/IR/AbstractionPattern.cpp

@@ -574,12 +574,14 @@ AbstractionPattern AbstractionPattern::getFunctionResultType() const {
         // If there's a single argument, abstract it according to its formal type
         // in the ObjC signature.
         unsigned callbackResultIndex = 0;
-        if (callbackErrorIndex && callbackResultIndex >= *callbackErrorIndex)
-          ++callbackResultIndex;
-        if (callbackErrorFlagIndex
-            && callbackResultIndex >= *callbackErrorFlagIndex)
-          ++callbackResultIndex;
-
+        for (auto index : indices(callbackParamTy->getParamTypes())) {
+          if (callbackErrorIndex && index == *callbackErrorIndex)
+            continue;
+          if (callbackErrorFlagIndex && index == *callbackErrorFlagIndex)
+            continue;
+          callbackResultIndex = index;
+          break;
+        }
         auto clangResultType = callbackParamTy
           ->getParamType(callbackResultIndex)
           .getTypePtr();

validation-test/compiler_crashers_2_fixed/rdar81617749.swift

@@ -25,9 +25,8 @@ func run(on object: PFXObject) async throws {
   // CHECK: performSingleBothy13
   print(try await object.performSingleBothy13()())
 
-  // Reenable with rdar://81625544
-  // CHECK  performSingleBothy21
-  //  print(try await object.performSingleBothy21()())
+  // CHECK: performSingleBothy21
+  print(try await object.performSingleBothy21()())
 
   // CHECK: performSingleBothy23
   print(try await object.performSingleBothy23()())