[IRGen] Put 'ret void' instead of unreachable for non swiftasync cc

If target doesn't support musttail (e.g. WebAssembly), the function
passed to coro.end.async can return control back to the caller.
So the frontend should emit 'ret void' instead of unreachable after the
coro.end.async intrinsic call to allow such situation.

(cherry picked from commit 671ce74)

Author: kateinoigakukun

lib/IRGen/GenCall.cpp

@@ -4949,7 +4949,15 @@ void irgen::emitAsyncReturn(
     arguments.push_back(arg);
 
   Builder.CreateIntrinsicCall(llvm::Intrinsic::coro_end_async, arguments);
-  Builder.CreateUnreachable();
+
+  if (IGF.IGM.AsyncTailCallKind == llvm::CallInst::TCK_MustTail) {
+    Builder.CreateUnreachable();
+  } else {
+    // If target doesn't support musttail (e.g. WebAssembly), the function
+    // passed to coro.end.async can return control back to the caller.
+    // So use ret void instead of unreachable to allow it.
+    Builder.CreateRetVoid();
+  }
 }
 
 void irgen::emitAsyncReturn(IRGenFunction &IGF, AsyncContextLayout &asyncLayout,

test/IRGen/async/non_musttail_target.sil

@@ -0,0 +1,17 @@
+// Ensure that IRGen don't emit unreachable after coro.end.async for targets that don't support musttail call.
+// RUN: %swift -disable-legacy-type-info -parse-stdlib -target wasm32-unknown-wasi %s -disable-llvm-optzns -disable-swift-specific-llvm-optzns -disable-objc-interop -module-name main -emit-ir -o - | %FileCheck %s
+// REQUIRES: concurrency
+// REQUIRES: CODEGENERATOR=WebAssembly
+
+sil_stage canonical
+
+import Builtin
+
+sil @test_simple : $@async () -> () {
+bb0:
+  %0 = tuple ()
+  return %0 : $()
+// CHECK:     call i1 (i8*, i1, ...) @llvm.coro.end.async
+// CHECK-NOT: unreachable
+// CHECK:     ret void
+}