Fix calling convention mismatch on Builtin.getCurrentAsyncTask (swiftlang#37008)

* Add explicit calling convention on builtin GetCurrentTask

This builtin function emits a call of swift_task_getCurrent, and user
code can declare the same name function with slightly different
signature at LLVM level (data size should be same) using @_silgen_name.
In that case, IRGen insert cast inst to cast the function to the
expected signature. But this cast inst drops calling convention info, so
call inst was emitted without swiftcc.

This patch changed to emit a call of swift_task_getCurrent with the
explicit calling convention.

* Add test case to ensure builtin function cc when conflict

Author: kateinoigakukun

lib/IRGen/GenCall.cpp

@@ -230,6 +230,7 @@ void IRGenFunction::setupAsync(unsigned asyncContextIndex) {
 llvm::Value *IRGenFunction::getAsyncTask() {
   auto call = Builder.CreateCall(IGM.getGetCurrentTaskFn(), {});
   call->setDoesNotThrow();
+  call->setCallingConv(IGM.SwiftCC);
   return call;
 }
 

test/IRGen/builtin_conflict.sil

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -emit-ir -parse-sil %s -parse-stdlib | %FileCheck %s
+
+import Builtin
+import Swift
+
+sil @entry : $@async @convention(thin) () -> () {
+bb0:
+  %0 = function_ref @swift_task_getCurrent : $@convention(thin) () -> @owned Optional<Builtin.NativeObject>
+  // CHECK: call swiftcc {{.*}} @swift_task_getCurrent
+  %1 = apply %0() : $@convention(thin) () -> @owned Optional<Builtin.NativeObject>
+  // CHECK-NEXT: call swiftcc {{.*}} @swift_task_getCurrent
+  %3 = builtin "getCurrentAsyncTask"() : $Builtin.NativeObject
+  return undef : $()
+}
+
+sil hidden_external @swift_task_getCurrent : $@convention(thin) () -> @owned Optional<Builtin.NativeObject>