[llvm-profdata] ProfileReader cleanup - preparation for MD5 refactoring - 3

Cleanup profile reader classes to prepare for complex refactoring as propsed in D147740, continuing D148872
This is patch 3/n. This patch changes the behavior of function offset table.

Previously when reading ExtBinary profile, the funcOffsetTable (map) is always populated, and in addition if the profile is CS, the orderedFuncOffsets (list) is also populated. However when reading the function samples, only one of the container is being used, never both, so it's a huge waste of time to populate both. Added logic to select which one to use, and completely skip reading function offset table if we are in tool mode (all function samples are to be read sequentially regardless)

Reviewed By: davidxl, wenlei

Differential Revision: https://reviews.llvm.org/D149124