[SampleFDO] Extract the code calling each section reader to readOneSection.

This is a followup of https://reviews.llvm.org/D66513. The code calling each
section reader should be put into a separate function (readOneSection), so
SampleProfileExtBinaryReader can override it. Otherwise, the base class
SampleProfileExtBinaryBaseReader will need to be aware of all different kinds
of section readers. That is not right.

Differential Revision: https://reviews.llvm.org/D66693

llvm-svn: 369919