[libc++] Allow picking Lit parameters from the config

Unlike parameters in litConfig.params, the config isn't shared across
all test suites. For example, if we want to enable exceptions in the
tests for libcxxabi, but not in the tests for libcxx, we can't set the
enable_exceptions parameter in the litConfig object, cause it will be
used by both. Instead, setting it inside the config object solves that
problem.