[profile] Add interface to get profile filename

Summary:
Add __llvm_profile_get_filename interface to get the profile filename,
which can be used for identifying which profile file belongs to an app
when multiple binaries are instrumented and dumping profiles into the
same directory. The filename includes the path.

Reviewers: davidxl

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D49529

llvm-svn: 337482