Implement LTOModule on top of IRObjectFile.

IRObjectFile provides all the logic for producing mangled names and getting
symbols from inline assembly.

LTOModule then adds logic for linking specific tasks, like constructing
llvm.compiler_user or extracting linker options from the bitcode.

The rule of the thumb is that IRObjectFile has the functionality that is
needed by both LTO and llvm-ar.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212349 91177308-0d34-0410-b5e6-96231b3b80d8