[lldb-tests] Force use of system stdlib for Objective-C test

The test TestObjCDirectMethods loads the Objective C runtime, which
doesn't work well with custom a libcxx, resulting in two copies of the
standard library being loaded at runtime.

Like what was done for `TestObjCExceptions`, this commit forces the
usage of the system's library instead. The minimum required Clang
version is set to the oldest Clang that can compile the libraries
available in the lldb-matrix bots.

Differential Revision: https://reviews.llvm.org/D136600