[llvm-profgen] Refactor PerfReader to allow different types of perf scripts

In order to support different types of perf scripts, this change tried to refactor `PerfReader` by adding the base class `PerfReaderBase` and current HybridPerfReader is derived from it for CS profile generation. Common functions like, passMM2PEvents, extract_lbrs, extract_callstack, etc. can be reused.

Next step is to add LBR only reader(for non-CS profile) and aggregated perf scripts reader(do a pre-aggregation of scripts).

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D107014