Small refactoring of CodeGen classes to fix naming of CodeGen, make LLVM logic common.

This PR makes the naming of our CodeGen classes a bit more logical and eliminates duplicated code in compiling for LLVM. I originally started on this while tracking down the optimize_module crash, which turned out to be something else, but I feel this is an improvement to the code and sets up some other changes as follows:

1) Adding a top-level CodeGen class which is independent of LLVM. This should make some parts of StmtCompiler cleaner and can make it clear that functionality such as function_takes_user_context is not connected to LLVM.

2) CodeGen_Posix should go away. It has nothing to do with Posix and can either be folded in to CodeGen_LLVM, or made into a helper class. (CodeGen_Allocation or some such.)

3) All LLVM specific codegen for GPU backends should be built on Halide IR so that we can do things like call GPU code from the C backend. (There is not much of a use case for doing this from C, but it makes a lot of sense for a JavaScript backend, which I hope we will have.) Hopefully this will allow getting rid of the templated CodeGen_GPU_Host classes.
IIRC, the no-longer-valid reason for calling this CodeGen_Posix is that it originally called malloc and free directly. Now it's more general than that, and handles anything with a notion of a heap and a stack (i.e. any CPU-like target). Wasn't sure what to name it then, not sure what to name it now. CodeGen_CPU?