[clang] Unreachable when -fwinx64-eh-unwindv2 is used on function with __finally block

[eleviant]

Steps to reproduce:

1. Create source file a.c

int bar();
void do_finalize();

int foo() {
  __try {
    while(1) {
      if (bar() == 42)
        break;
    }
  } __finally {
    do_finalize();
  }

  return 42;

}

2. Compile with clang

clang-cl.exe -cc1 -triple x86_64-pc-windows-msvc19.50.35730 -emit-obj -funwind-tables=2 -target-cpu x86-64  -tune-cpu generic -fwinx64-eh-unwindv2 -cfguard   -fms-extensions  -o a.obj -x c a.c

Unreachable is fired in X86WinEHUnwindV2::runOnMachineFunction, because it's expecting frame SEH_PushReg to be in prolog, not in finally block