[XRay][profiler] Part 3: Profile Collector Service

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

This patch implements a centralised collector for `FunctionCallTrie`
instances, associated per thread. It maintains a global set of trie
instances which can be retrieved through the XRay API for processing
in-memory buffers (when registered). Future changes will include the
wiring to implement the actual profiling mode implementation.

This central service provides the following functionality:

*  Posting a `FunctionCallTrie` associated with a thread, to the central
   list of tries.

*  Serializing all the posted `FunctionCallTrie` instances into
   in-memory buffers.

*  Resetting the global state of the serialized buffers and tries.

Depends on D45757.

Reviewers: echristo, pelikan, kpw

Reviewed By: kpw

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D45758

llvm-svn: 333624