Refactored driver logic for CodeGen into LLVMCodeGenWriter.  This ASTConsumer layers on top of LLVMCodeGen (another existing ASTConsumer) to emit bitcode files to disk.  This layering takes this logic out of clang.cpp and puts it directly into the ASTConsumer interface.  The benefit is that now --emit-llvm works with both serialized ASTs and regular source files.

llvm-svn: 54364