[CSSPGO][llvm-profgen] Fix external address issues of perf reader (leading external LBR part)

We can have the sampling just hit into the external addresses, in that case, both the top stack frame and the latest LBR target are external addresses. For example:
```
	        ffffffff
 0x4006c8/0xffffffff/P/-/-/0  0x40069b/0x400670/M/-/-/0

 	          ffffffff
	          40067e
0xffffffff/0xffffffff/P/-/-/0  0x4006c8/0xffffffff/P/-/-/0  0x40069b/0x400670/M/-/-/0
```
Before we will ignore the entire samples. However, we found there exists some internal LBRs in the remaining part of sample, the range between them is still a valid range, we will lose some valid LBRs. Those LBRs will be unwinded based on a empty(context-less) call stack.

This change tries to fix it, instead of ignoring the entire sample, we only ignore the leading external addresses.

Note that the first outgoing LBR is useful since there is a valid range between it's source and next LBR's target.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D115538