[clang-doc] Attaching a name to reference data

[clang-doc] Attaching a name to reference data

This adds the name of the referenced decl, in addition to its USR, to
the saved data, so that the backend can look at an info in isolation and
still be able to construct a human-readable name for it.

Differential Revision: https://reviews.llvm.org/D46281

llvm-svn: 331539