[XRay] Set an explicit dependency on libc++ when needed

When XRay is being built as part of the just built compiler together
with libc++ as part of the runtimes build, we need an explicit
dependency from XRay to libc++ to make sure that the library is
available by the time we start building XRay.

Differential Revision: https://reviews.llvm.org/D48113

llvm-svn: 334575