Reland "[lldb] Use just-built libcxx for tests when available"

This commit improves upon cc0b5ebf, which added support for
specifying which libcxx to use when testing LLDB. That patch honored
requests by tests that had `USE_LIBCPP=1` defined in their makefiles.
Now, we also use a non-default libcxx if all conditions below are true:

1. The test is not explicitly requesting the use of libstdcpp
(USE_LIBSTDCPP=1).
2. The test is not explicitly requesting the use of the system's
library (USE_SYSTEM_STDLIB=1).
3. A path to libcxx was either provided by the user through CMake flags
or libcxx was built together with LLDB.

Condition (2) is new and introduced in this patch in order to support
tests that are either:

* Cross-platform (such as API/macosx/macCatalyst and
API/tools/lldb-server). The just-built libcxx is usually not built for
platforms other than the host's.
* Cross-language (such as API/lang/objc/exceptions). In this case, the
Objective C runtime throws an exceptions that always goes through the...