[CMake] Unify and relayer testing

This patch restructures part of LLDB's testing configuration:

1. I moved the test dependencies up the chain so every dotest dependency
   becomes a lit dependency as well. It wouldn't make sense for dotest to
   have other dependencies when it's being run by lit. Lit on the other
   hand can still specify extra dependencies.

2. I replaced as much generator expressions with variables as possible.
   This is consistent with the rest of LLVM and doesn't break generators
   that support multiple targets (MSVC, Xcode). This wasn't a problem
   before, but now we need to expand the dotest arguments in the lit
   configuration and there's only one test suite even with multiple
   targets.

3. I moved lldb-dotest into it's own directory under utils since there's
   no need anymore for it to located under `test/`.

Differential revision: https://reviews.llvm.org/D46334

llvm-svn: 331463