[llvm-profdata] Add option to cap profile output size

Allow user to specify `--output-size-limit=n` to cap the size of generated profile to be strictly under n. Functions with the lowest total sample count are dropped first if necessary. Due to using a heuristic, excessive functions may be dropped to satisfy the size requirement

Reviewed By: snehasish

Differential Revision: https://reviews.llvm.org/D139603