[perf2bolt] Optimize memory usage in perf2bolt

Summary:
While converting perf profile, we only need CFG for functions that were
profiled and can skip building CFG for the rest. This saves us some
processing time and memory.

Breakdown processing of perf.data into two steps. The first
step parses the data, saves it in intermediate format, and marks
functions with the profile. The second step attributes the profile to
functions with CFG. When we disassemble and build CFG for functions in
aggregate-only mode, we skip functions without the profile.

(cherry picked from FBD13706697)