[XRay][darwin] Initial XRay in Darwin Support

Summary:
This change implements the changes required in both clang and
compiler-rt to allow building XRay-instrumented binaries in Darwin. For
now we limit this to x86_64. We also start building the XRay runtime
library in compiler-rt for osx.

A caveat to this is that we don't have the tests set up and running
yet, which we'll do in a set of follow-on changes.

This patch uses the monorepo layout for the coordinated change across
multiple projects.

Reviewers: kubamracek

Subscribers: mgorny, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D39114

llvm-svn: 317875