[CMake] Track test dependencies with add_lldb_test_dependency

I often use `ninja lldb-test-deps` to build all the test dependencies
before running a subset of the tests with `lit --filter`. This
functionality seems to break relatively often because test dependencies
are tracked in an ad-hoc way acrooss cmake files. This patch adds a
helper function `add_lldb_test_dependency` to unify test dependency
tracking by adding dependencies to lldb-test-deps.

Differential revision: https://reviews.llvm.org/D68612

llvm-svn: 373996