[lldb] Make the API, Shell and Unit tests independent lit test suites

Make the API, Shell and Unit tests independent lit test suites. This
allows us to specify different dependencies and skip rebuilding all the
unit test (which is particularly expensive) when running check-lldb-api
or check-lldb-shell.

This does not change the autogenerated targets such as
check-lldb-shell-driver or the top level check-lldb target, which all
continue to work as before.

Differential revision: https://reviews.llvm.org/D98842