[XRay][llvm+clang] Consolidate attribute list files

Summary:
This change consolidates the always/never lists that may be provided to
clang to externally control which functions should be XRay instrumented
by imbuing attributes. The files follow the same format as defined in
https://clang.llvm.org/docs/SanitizerSpecialCaseList.html for the
sanitizer blacklist.

We also deprecate the existing `-fxray-instrument-always=` and
`-fxray-instrument-never=` flags, in favour of `-fxray-attr-list=`.

This fixes http://llvm.org/PR34721.

Reviewers: echristo, vlad.tsyrklevich, eugenis

Reviewed By: vlad.tsyrklevich

Subscribers: llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D45357

llvm-svn: 329543