clover/llvm: Declare compiler instance at top level and pass down as argument.

clover/llvm: Declare compiler instance at top level and pass down as argument.

This allows simplifying the interface of compile_llvm() because it no
longer needs to read out and return the optimization level and address
space map from the compiler instance.  Instead declare the compiler
instance at the top level so that both properties are available
directly.

Reviewed-by: Serge Martin <edb+mesa@sigluy.net>
Tested-by: Jan Vesely <jan.vesely@rutgers.edu>