[InstrProf] Add support for dead_strip+live_support functionality

On Darwin, marking a section as "regular,live_support" means that a
symbol in the section should only be kept live if it has a reference to
something that is live. Otherwise, the linker is free to dead-strip it.

Turn this functionality on for the __llvm_prf_data section.

This means that counters and data associated with dead functions will be
removed from dead-stripped binaries. This will result in smaller
profiles and binaries, and should speed up profile collection.

Tested with check-profile, llvm-lit test/tools/llvm-{cov,profdata}, and
check-llvm.

Differential Revision: https://reviews.llvm.org/D25456

llvm-svn: 283947