[lldb] Fix missing dependency on libc++ from LLDB test suite on non-Darwin platforms

Right now we only set a dependency on libc++ when the host is Darwin, which
means that libc++ in the build directory is in some undefined state when running
the test suite (it can be fully built, out-of-date or missing). Depending on
whether we have a system libc++ (which LLDB also supports running the libc++
tests against), the outcome is that we sometimes skip the libc++ tests or we run
the tests against a mix of ToT-libc++/system-libc++ (e.g., we compile against
the ToT-libc++ headers and link against the system libc++ library).

This can be demonstrated via `export LIT_FILTER=TestDataFormatterLibcxxSet ninja
check-lldb-api` (or any other libc++ test) and then gradually building parts of
libc++ in the same build (which will slowly change the test behaviour from
`UNSUPPORTED` to various failures to passing depending on how much of libcxx is
built at test time).

Note that this effectively re-enables the (unintentionally) disabled libc++
formatter tests on Linux. Don't revert this if it breaks a libc++ LLDB test,
instead please @skipIf decorate the failing test (as it was probably already
failing before this commit).

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D111981

Author: Teemperor

lldb/test/CMakeLists.txt

@@ -135,9 +135,16 @@ if(TARGET clang)
             "`LLDB_INCLUDE_TESTS=OFF`.")
         endif()
       endif()
-      add_lldb_test_dependency(cxx)
     endif()
   endif()
+
+  # Add libc++ dependency for libc++-specific tests. This is an optional
+  # dependency as it's also possible to run the libc++ tests against the libc++
+  # installed on the system.
+  if (TARGET cxx)
+    add_lldb_test_dependency(cxx)
+  endif()
+
 endif()
 
 if (LLDB_BUILT_STANDALONE)