[IRGen] Fix SignatureExpansion::expandAsyncReturnType()

rdar://141962253

This fixes an issue that caused parameters to be dropped when a typed error in an async closure was too big to be returned directly.

Author: drexin

lib/IRGen/GenCall.cpp

@@ -2142,9 +2142,9 @@ void SignatureExpansion::expandAsyncReturnType() {
       } else {
         ParamIRTypes.push_back(combined.combinedTy);
       }
+      addErrorResult();
+      return;
     }
-    addErrorResult();
-    return;
   }
 
   if (native.requiresIndirect() || native.empty()) {

test/IRGen/typed_throws.swift

@@ -285,3 +285,13 @@ func callClosureAsync<T>(t: T) async {
     return t
   }
 }
+
+enum LargeError: Error {
+  case x
+  case y(Int64, Int64, Int64, Int64, Int64)
+}
+
+// Used to crash the compiler because
+func callClosureAsyncIndirectError(f: () async throws(LargeError) -> Int) async throws(LargeError) -> Int {
+  return try await f()
+}