[XRay][profiler] Part 4: Profiler Mode Wiring

Summary:
This is part of the larger XRay Profiling Mode effort.

This patch implements the wiring required to enable us to actually
select the `xray-profiling` mode, and install the handlers to start
measuring the time and frequency of the function calls in call stacks.
The current way to get the profile information is by working with the
XRay API to `__xray_process_buffers(...)`.

In subsequent changes we'll implement profile saving to files, similar
to how the FDR and basic modes operate, as well as means for converting
this format into those that can be loaded/visualised as flame graphs. We
will also be extending the accounting tool in LLVM to support
stack-based function call accounting.

We also continue with the implementation to support building small
histograms of latencies for the `FunctionCallTrie::Node` type, to allow
us to actually approximate the distribution of latencies per function.

Depends on D45758 and D46998.

Reviewers: eizan, kpw, pelikan

Reviewed By: kpw

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D44620

llvm-svn: 334469