[BOLT] Initial experimental instrumentation pass

Summary:
An instrumentation pass that modifies the input binary to
generate a profile after execution finishes. It modifies branches to
increment counters stored in the process memory and injects a new
function that dumps this data to an fdata file, readable by BOLT.

This instrumentation is experimental and currently uses a naive
approach where every branch is instrumented. This is not ideal for
runtime performance, but should be good enough for us to
evaluate/debug LBR profile quality against instrumentation.

Does not support instrumenting indirect calls yet, only direct
calls, direct branches and indirect local branches.

(cherry picked from FBD15998096)