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

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