[lit] Print substitutions with --show-suites

We already print available features, and it can be useful to print
substitutions as well since those are a pretty fundamental part of
a test suite. We could also consider printing other things like the
test environment, however the need doesn't appear to be as strong.

As a fly-by fix, we also always print available features, even when
there are none.

Before:

  $ lit -sv libcxx/test --show-suites
  -- Test Suites --
    libc++ - 6350 tests
      Source Root: [...]
      Exec Root  : [...]
      Available Features : -faligned-allocation -fsized-deallocation [...]

After:

  $ lit -sv libcxx/test --show-suites
  -- Test Suites --
    libc++ - 6350 tests
      Source Root: [...]
      Exec Root  : [...]
      Available Features: -faligned-allocation -fsized-deallocation [...]
      Available Substitutions: %{build_module} => [...]
                               %{build} => %{cxx} -o [...]

Differential Revision: https://reviews.llvm.org/D77818

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