[llvm-profgen] Trim cold function profiles for non-CS AutoFDO

This change allows to trim the profile if it's considered to be cold for baseline AutoFDO. We reuse the cold threshold from `ProfileSummaryBuilder::getColdCountThreshold(..)` which can be set by percent(--profile-summary-cutoff-cold) or by value(--profile-summary-cold-count).

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D113785