Orc: Simplify RPC naming system by using function-local statics

The existing scheme of class template static members for Name and
NameMutex is a bit verbose, involves global ctors (even if they're cheap
for string and mutex, still not entirely free), and (importantly/my
immediate motivation here) trips over a bug in LLVM's modules
implementation that's a bit involved (hmm, sounds like Mr. Smith has a
fix for the modules thing - but I'm still inclined to commit this patch
as general goodness).

llvm-svn: 353241