This is the third of a multi-part change to implement subcommands for
the llvm-xray tool.
Here we define the account subcommand which does simple function call
accounting, generating basic statistics on function calls we find in an
XRay log/trace. We support text output and csv output for this
Part of this tool will later be turned into a library that could be used for
basic function call accounting.
Depends on D24376.