[BOLT] Major overhaul of profiling in BOLT

Summary:
Profile reading was tightly coupled with building CFG. Since I plan
to move to a new profile format that will be associated with CFG
it is critical to decouple the two phases.

We now have read profile right after the cfg was constructed, but
before it is "canonicalized", i.e. CTCs will till be there.

After reading the profile, we do a post-processing pass that fixes
CFG and does some post-processing for debug info, such as
inference of fall-throughs, which is still required with the current
format.

Another good reason for decoupling is that we can use profile with
CFG to more accurately record fall-through branches during
aggregation.

At the moment we use "Offset" annotations to facilitate location
of instructions corresponding to the profile. This might not be
super efficient. However, once we switch to the new profile format
the offsets would be no longer needed. We might keep them for
the aggregator, but if we have to trust LBR data that might
not be strictly necessary.

I've tried to make changes while keeping backwards compatibly. This makes
it easier to verify correctness of the changes, but that also means
that we lose accuracy of the profile.

Some refactoring is included.

Flag "-prof-compat-mode" (on by default) is used for bug-level
backwards compatibility. Disable it for more accurate tracing.

(cherry picked from FBD6506156)