[XRay][compiler-rt][NFC] Add example always/never instrument files.

Summary:
This change introduces two files that show exaples of the
always/never instrument files that can be provided to clang. We don't
add these as defaults yet in clang, which we can do later on (in a
separate change).

We also add a test that makes sure that these apply in the compiler-rt
project tests, and that changes in clang don't break the expectations in
compiler-rt.

Reviewers: pelikan, kpw

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34669

llvm-svn: 306502