LTO: Simplify merged module ownership.

This change moves LTOCodeGenerator's ownership of the merged module to a
field of type std::unique_ptr<Module>. This helps simplify parts of the code
and clears the way for the module to be consumed by LLVM CodeGen (see D12132
review comments).

Differential Revision: http://reviews.llvm.org/D12205

llvm-svn: 245891