[lldb] Explicitly set libcxx paths when USE_SYSTEM_STDLIB is provided

[lldb] Explicitly set libcxx paths when USE_SYSTEM_STDLIB is provided

For tests marked as "USE_SYSTEM_STDLIB", the expectation is that the
system's standard library should be used. However, the implementation of
this flag is such that we simply don't pass _any_ libcxxx-related flags
to Clang; in turn, Clang will use its defaults.

For a Clang/Libcxx pair compiled together, Clang defaults to:
1. The headers of the sibling libcxx.
2. The libraries of the system.

This mismatch is actually a bug in the driver; once fixed, however, (2)
would point to the sibling libcxx as well, which is _not_ what test
authors intended with the USE_SYSTEM_STDLIB flag.

As such, this patch explicitly sets a path to the system's libraries.
This change is done only in Apple platforms so that we can test this
works in this case first.

Differential Revision: https://reviews.llvm.org/D146714