[Profiling] Add a -sparse mode to llvm-profdata merge

Add an option to llvm-profdata merge for writing out sparse indexed
profiles. These profiles omit InstrProfRecords for functions which are
never executed.

Differential Revision: http://reviews.llvm.org/D16727

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259258 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed