[Refactoring] Use known alternative in async refactoring if one exists

Update the async refactorings to use the name from the async alternative
if one is known, rather than always assuming the name matches the
synchronous function.

Note that this could also be used to determine whether results remain
optional or not, but that has been left for a future patch.

Resolves rdar://80612521

Author: bnbarham

include/swift/AST/Decl.h

@@ -6210,7 +6210,7 @@ class AbstractFunctionDecl : public GenericContext, public ValueDecl {
   /// handler if \p asyncAlternative is not set (with the same conditions on
   /// its type as above).
   Optional<unsigned> findPotentialCompletionHandlerParam(
-      AbstractFunctionDecl *asyncAlternative = nullptr) const;
+      const AbstractFunctionDecl *asyncAlternative = nullptr) const;
 
   /// Determine whether this function is implicitly known to have its
   /// parameters of function type be @_unsafeSendable.

lib/AST/Decl.cpp

@@ -7092,7 +7092,7 @@ static bool isPotentialCompletionHandler(const ParamDecl *param) {
 }
 
 Optional<unsigned> AbstractFunctionDecl::findPotentialCompletionHandlerParam(
-    AbstractFunctionDecl *asyncAlternative) const {
+    const AbstractFunctionDecl *asyncAlternative) const {
   const ParameterList *params = getParameters();
   if (params->size() == 0)
     return None;
@@ -7175,7 +7175,7 @@ Optional<unsigned> AbstractFunctionDecl::findPotentialCompletionHandlerParam(
 
     // The next original param should match the current async, so don't
     // increment the async index
-    potentialParam = asyncParamIndex;
+    potentialParam = paramIndex;
     paramIndex++;
   }
   return potentialParam;