[lldb] Use the just-built libc++ for testing the LLDB data formatters

Make sure we use the libc++ from the build dir. Currently, by passing
-stdlib=libc++, we might pick up the system libc++. This change ensures
that if LLVM_LIBS_DIR is set, we try to use the libc++ from there.

Differential revision: https://reviews.llvm.org/D129166