[XRay] Implement the `llvm-xray account` subcommand

Summary:
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
subcommand.

This change also supports sorting, summing, and filtering the top N
results.

Part of this tool will later be turned into a library that could be used
for basic function call accounting.

Depends on D24376.

Reviewers: dblaikie, echristo

Subscribers: mehdi_amini, dberris, beanz, llvm-commits

Differential Revision: https://reviews.llvm.org/D24377

llvm-svn: 291749