[XRay][clang] Allow imbuing arg1 logging attribute via -fxray-always-instrument=

Summary:
This change allows us to add arg1 logging support to functions through
the special case list provided through -fxray-always-instrument=. This
is useful for adding arg1 logging to functions that are either in
headers that users don't have control over (i.e. cannot change the
source) or would rather not do.

It only takes effect when the pattern is matched through the "fun:"
special case, as a category. As in:

  fun:*pattern=arg1

Reviewers: pelikan, rnk

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33392

llvm-svn: 303719