[XRay] [clang] Allow logging the first argument of a function call.

Summary:
Functions with the "xray_log_args" attribute will tell LLVM to emit a special
XRay sled for compiler-rt to copy any call arguments to your logging handler.

Reviewers: dberris

Reviewed By: dberris

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D29704

llvm-svn: 296999