PGO: llvm-profdata: tool for merging profiles

Introducing llvm-profdata, a tool for merging profile data generated by
PGO instrumentation in clang.

- The name indicates a file extension of <name>.profdata.  Eventually
  profile data output by clang should be changed to that extension.

- llvm-profdata merges two profiles.  However, the name is more general,
  since it will likely pick up more tasks (such as summarizing a single
  profile).

- llvm-profdata parses the current text-based format, but will be
  updated once we settle on a binary format.

<rdar://problem/15949645>