Profile: Add a library for the instrumentation based profiling format

This provides a library to work with the instrumentation based
profiling format that is used by clang's -fprofile-instr-* options and
by the llvm-profdata tool. This is a binary format, rather than the
textual one that's currently in use.

The tests are in the subsequent commits that use this.
llvm-profdata: Use the Profile library, implement show and generate

This replaces the llvm-profdata tool with a version that uses the
recently introduced Profile library. The new tool has the ability to
generate and summarize profdata files as well as merging them.