Add C++ dependencies to xray runtime

Summary:
Depends on D21982 which implements the in-memory logging implementation of the
XRay runtime. These additional changes also depends on D20352 which adds the
bulk of XRay flags/dependencies when using the `-fxray-instrument` flag from
Clang.

Reviewers: echristo, rnk, aaron.ballman

Subscribers: mehdi_amini, cfe-commits

Differential Revision: http://reviews.llvm.org/D21983

llvm-svn: 275368