[XRay][compiler-rt] Document and update the XRay Logging API

Summary:
In this patch we document the requirements for implementations that want
to install handlers for the dynamically-controlled XRay "framework".
This clarifies what the expectations are for implementations that
want to install their handlers using this API (similar to how the FDR
logging implementation does so). It also gives users some guarantees on
semantics for the APIs.

If all goes well, users can decide to use the XRay APIs to control the
tracing/logging at the application level, without having to depend on
implementation details of the installed logging implementation. This
lets users choose the implementation that comes with compiler-rt, or
potentially multiple other implementations that use the same APIs.

We also add one convenience function (__xray_remove_log_impl()) for
explicitly removing the currently installed log implementation.

Reviewers: kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32579

llvm-svn: 301784