[XRay][clang] Add a flag to enable/disable linking XRay deps explicitly

Summary:
This change introduces `-fxray-link-deps` and `-fnoxray-link-deps`. The
`-fnoxray-link-deps` allows for directly controlling which specific XRay
runtime to link. The default is for clang to link the XRay runtime that
is shipped with the compiler (if there are any), but users may want to
explicitly add the XRay dependencies from other locations or other
means.

Reviewers: eizan, echristo, chandlerc

Reviewed By: eizan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45354

llvm-svn: 329376