[XRay] Do not depend on C++ stdlib for XRay builds

Summary:
Now that XRay doesn't require a runtime dependency on a C++ standard
library, we remove that dependency from the clang linker flags.

Reviewers: saugustine, pelikan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31313

llvm-svn: 298670