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

Summary:
Functions with the "xray-log-args" attribute will have a special XRay sled kind
emitted, for compiler-rt to copy any call arguments to your logging handler.

For practical and performance reasons, only the first argument is supported, and
only up to 64 bits.

Reviewers: dberris

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29702

llvm-svn: 296998