[XRay][clang] Consolidate runtime and link-time flag processing (NFC)

Summary:
This change fixes http://llvm.org/PR36985 to define a single place in
CommonArgs.{h,cpp} where XRay runtime flags and link-time dependencies
are processed for all toolchains that support XRay instrumentation. This
is a refactoring of the same functionality spread across multiple
toolchain definitions.

Reviewers: echristo, devnexen, eizan

Reviewed By: eizan

Subscribers: emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D45243

llvm-svn: 329372