[llvm-profgen] Refactor and better diagnostics

This change contains diagnostics improvments, refactoring and preparation for consuming perf data directly.

Diagnostics:
 - We now have more detailed diagnostics when no mmap is found.
 - We also print warning for abnormal transition to external code.

Refactoring:
 - Simplify input perf trace processing to only allow a single input file. This is because 1) using multiple input perf trace (perf script) is error prone because we may miss key mmap events. 2) the functionality is not really being used anyways.
 - Make more functions private for Readers, move non-trivial definitions out of header. Cleanup some inconsistency.
 - Prepare for consuming perf data as input directly.

Differential Revision: https://reviews.llvm.org/D110729