[AutoFDO] Use getHeadSamplesEstimate instead of getTotalSamples to compute...

[AutoFDO] Use getHeadSamplesEstimate instead of getTotalSamples to compute profile callsite staleness

Fix two issues for profile staleness report.

1) It should be more accurate to use the sum of all entry count(`getHeadSamplesEstimate`) for the callsite samples than the total samples, since even the top-level callsite is mismatched, it does affect the inlining but it can still be merged into base profile and used later.

2) I accidentally missed to persist the num of mismatched callsite into binary.

Also added the asm testing to test the decoding of the section.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D140063