[XRay] Implement `llvm-xray convert` -- trace file conversion

This is the second part of a multi-part change to define additional
subcommands to the `llvm-xray` tool.

This change defines a conversion subcommand to take XRay log files, and
turns them from one format to another (binary or YAML). This currently
only supports the first version of the log file format, defined in the
compiler-rt runtime.

Depends on D21987.

Reviewers: dblaikie, echristo

Subscribers: mehdi_amini, dberris, beanz, llvm-commits

Differential Revision: https://reviews.llvm.org/D24376

llvm-svn: 291529