[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the test suite (it can be fully built, out-of-date or missing).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

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