swift234

Rename OptimizeARC library => LLVMPasses library.

OptimizeARC does not only contain an optimize arc pass: the library also
includes aa. What this really is a repository of the extra passes and
infrastructure that we inject into LLVM. Thus LLVMPasses is a more descriptive
name. It also matches SILPasses.

I also taught lit how to use the new llvm-opt driver for running swift llvm
passes through opt without having to remember how to setup the dynamic swift
llvm pass dylib. You can use this in lit tests by using the substitution
%llvm-opt.

Swift SVN r21654