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

Summary:
Functions with the LOG_ARGS_ENTRY sled kind at their beginning will be handled
in a way to (optionally) pass their first call argument to your logging handler.

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

Reviewers: javed.absar, dberris

Reviewed By: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29703

llvm-svn: 297000