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.