[lldb] Correctly add runtime test dependencies

When a runtime is enabled through LLVM_ENABLE_RUNTIMES, it is loaded
after other projects (i.e. after LLDB). This means that the
corresponding targets don't exist when LLDB is configured. To support
runtimes being enable this way, we need to check if they're listed in
LLVM_ENABLE_RUNTIMES. For runtimes being enabled as projects (i.e.
through LLVM_ENABLE_RUNTIMES) we need to check for the target.

This patch unifies things and correctly adds compiler-rt and libcxx as
test dependencies in both scenarios.