clover/llvm: Split compilation and linking.

clover/llvm: Split compilation and linking.

Split the work previously done by compile_program_llvm() into
compile_program() (which simply runs the front-end and serializes the
resulting LLVM IR) and link_program() (which takes care of everything
else down to binary codegen).

[ Serge Martin: allow LLVM IR dump after compilation ]

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