[ORC] Guarantee unique JITDylib names in lli, add usage notes to createJITDylib.

JITDylibs should have unique names. This patch adds code to lli to respect this
invariant (by refering to the exist JITDylib if a -jd <name> option is specified
more than once). It also adds usage notes to the doxygen comment for
createJITDylib method in ExecutionSession and LLJIT.

http://llvm.org/PR41937

llvm-svn: 361322