[lldb-tests] Remove dubious standard library flag

The test currently sets `USE_LIBSTDCPP = 0`, which is curious given the
behavior of `and` and `or` in Makefiles (the contents of the variables
are not important). In particular, this causes the tests to not use the
standard libraries appropriately.

To capture the actual intent of the test, we're changing this to
`USE_LIBCXX=1`.

Differential Revision: https://reviews.llvm.org/D136171