[llvm-profgen] Ignore the whole trace with the leading external branch

The first LBR entry can be an external branch, we should ignore the whole trace.

```
     7f7448e889e4 0x7f7448e889e4/0x7f7448e88826/P/-/-/1  0x7f7448e8899f/0x7f7448e889d8/P/-/-/4  ...
```

Reviewed By: wenlei, hoy

Differential Revision: https://reviews.llvm.org/D111749