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