[SampleFDO][NFC] Detach SampleProfileLoader from SampleCoverageTracker

This patch detaches SampleProfileLoader from class
SampleCoverageTracker. We plan to move SampleProfileLoader
to a template class. This would remain SampleCoverageTracker
as a class.
Also make callsiteIsHot() as a file static function.

Differential Revision: https://reviews.llvm.org/D95823