Add necessary support for storing code-model to module IR.

Currently the code-model does not get saved in the module IR, so if a
code model is specified when compiling with LTO, it gets lost and is
not propagated properly to LTO. This patch does what is necessary in
the front end to pass the code-model to the module, so that the back
end can store it in the Module .

Differential Revision: https://reviews.llvm.org/D52323

llvm-svn: 342758