[Dexter] Require lldb-dap for Dexter and log when Dexter is disabled (#206734)
For some time, Dexter tests have been run using lldb-dap when it is
available rather than just lldb/lldb-server. However, the cross project
test dependencies have not been updated since then, meaning that
lldb-dap is not automatically built by check-cross-project. The
Dexter-specific lit config then skips the Dexter tests if lldb-dap is
unavailable, which leads to some check-cross-project builds never
running the Dexter tests. This patch adds lldb-dap to the build
dependencies, and also adds a small log message to inform when the
Dexter tests are skipped; most cases where we skip the Dexter test were
already logged, and this addition expands that to cover all cases.