[SampleFDO] New hierarchical discriminator for FS SampleFDO (ProfileData part)

This patch was split from https://reviews.llvm.org/D102246
[SampleFDO] New hierarchical discriminator for Flow Sensitive SampleFDO
This is mainly for ProfileData part of change. It will load
FS Profile when such profile is detected. For an extbinary format profile,
create_llvm_prof tool will add a flag to profile summary section.
For other format profiles, the users need to use an internal option
(-profile-isfs) to tell the compiler that the profile uses FS discriminators.

This patch also simplified the bit API used by FS discriminators.

Differential Revision: https://reviews.llvm.org/D103041