[libc++] Rename check-cxx-deps to cxx-test-depends for consistency

Several subprojects have targets that do the same thing, and they all
follow the same naming convention: llvm-test-depends, clang-test-depends,
lld-test-depends, etc.

This makes libc++ consistent with other LLVM projects.
Thanks to Duncan Exon Smith for noticing and suggesting the change.

Differential Revision: https://reviews.llvm.org/D94499