Add an "-object-cache-dir=<string>" option to LLI. This option specifies the
root path to which object files managed by the LLIObjectCache instance should be
written. This option defaults to "", in which case objects are cached in the
same directory as the bitcode they are derived from.

The load-object-a.ll test has been rewritten to use this option to support
testing in environments where the test directory is not writable.