[SampleFDO] Third Try: Refactor SampleProfile.cpp

Apply the patch for the third time after fixing buildbot failures.

Refactor SampleProfile.cpp to use the core code in CodeGen.
The main changes are:
(1) Move SampleProfileLoaderBaseImpl class to a header file.
(2) Split SampleCoverageTracker to a head file and a cpp file.
(3) Move the common codes (common options and callsiteIsHot())
to the common cpp file.
(4) Add inline keyword to avoid duplicated symbols -- they will
be removed later when the class is changed to a template.

Differential Revision: https://reviews.llvm.org/D96455