Fix the bug when SampleProfileWriter writes out number of callsites.

Summary: As we support multiple callsites for the same location, we need to traverse all locations to get the number of callsites.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D36246

llvm-svn: 309907