[AutoFDO] Use flattened profiles for profile staleness metrics

For profile staleness report, before it only counts for the top-level function samples in the nested profile, the samples in the inlinees are ignored. This could affect the quality of the metrics when there are heavily inlined functions. This change adds a feature to flatten the nested profile and we're changing to use flatten profile as the input for stale profile detection and matching.
Example for profile flattening:

```
Original profile:
_Z3bazi:20301:1000
 1: 1000
 3: 2000
 5: inline1:1600
   1: 600
   3: inline2:500
     1: 500

Flattened profile:
_Z3bazi:18701:1000
 1: 1000
 3: 2000
 5: 600 inline1:600
inline1:1100:600
 1: 600
 3: 500 inline2: 500
inline2:500:500
 1: 500
```
This feature could be useful for offline analysis, like understanding the hotness of each individual function. So I'm adding the support to `llvm-profdata merge` under `--gen-flattened-profile`.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D146452