[XRay][tools] Updated stacks tool with flamegraph output.

Summary:
As the first step to allow analysis and visualization of xray collected data,
allow using the llvm-xray stacks tool to emit a complete listing of stacks in
the format consumable by a flamegraph tool.

Possible follow up formats include chrome trace viewer format and sql load
files.

As a POC, I'm able to generate flamegraphs of an xray instrumented llc compiling
hello world.

Reviewers: dberris, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38650

llvm-svn: 315635