[Windows SEH] Fix catch+return crash for Windows -EHa

[pengfei]

Summary

This change also turns on -fasync-exceptions by default under -EHa
option due to the backend patch merged.

[pengfei]

I checked no difference in codegen between /EHa and /EHsc in MSVC https://godbolt.org/z/rcMaP6Wse. So it should be correct for clang as well.
We should land it ASAP since it fixes a crash case.

[LuoYuanke]

LGTM, pls wait for 1 or 2 days in case there are comments from others

clang/lib/CodeGen/CGCleanup.cpp

// the cleanup block and then try to clean up after ourselves.
       }
 
       // II.  Emit the entry block.  This implicitly branches to it if
       // we have fallthrough.  All the fixups and existing branches
       // should already be branched to it.
       EmitBlock(NormalEntry);
 
       // intercept normal cleanup to mark SEH scope end
-      if (IsEHa) {
+      if (IsEHa && getInvokeDest()) {
         if (Personality.isMSVCXXPersonality())
           EmitSehCppScopeEnd();
         else
           EmitSehTryScopeEnd();
       }
 
       // III.  Figure out where we're going and build the cleanup
       // epilogue.

clang/lib/Driver/ToolChains/Clang.cpp

else
                                                            << "'/Zi', '/Z7'";
   }
 
   EHFlags EH = parseClangCLEHFlags(D, Args);
   if (!isNVPTX && (EH.Synch || EH.Asynch)) {
     if (types::isCXX(InputType))
       CmdArgs.push_back("-fcxx-exceptions");
     CmdArgs.push_back("-fexceptions");
+    if (EH.Asynch)
+      CmdArgs.push_back("-fasync-exceptions");
   }
   if (types::isCXX(InputType) && EH.Synch && EH.NoUnwindC)
     CmdArgs.push_back("-fexternc-nounwind");
 
   // /EP should expand to -E -P.
   if (Args.hasArg(options::OPT__SLASH_EP)) {
     CmdArgs.push_back("-E");
     CmdArgs.push_back("-P");

clang/test/CodeGen/windows-seh-EHa-CppCatchReturn.cpp

+// RUN: %clang_cc1 -triple x86_64-windows -fasync-exceptions -fcxx-exceptions -fexceptions -fms-extensions -x c++ -Wno-implicit-function-declaration -S -emit-llvm %s -o - | FileCheck %s
+
+// CHECK: define dso_local void @"?foo@@YAXXZ
+// CHECK: invoke void @llvm.seh.try.begin()
+// CHECK-NOT: llvm.seh.scope.begin
+// CHECK-NOT: llvm.seh.scope.end
+
+// FIXME: Do we actually need llvm.seh.scope*?
+void foo() {
+  try {}
+  catch (...) {
+  return;
+  }
+}