expose LTO

[bscarlet]

No description provided.

[bscarlet]

LLVM LTO appears to consist of two layers. At the core is the capability to combine Modules. On top of that layer is a complex API in libLTO designed to be glued into a linker, to support linking a mixture of bitcode and native object files, apparently primarily optimizing the combination of the bitcode files. I've split the former (the core functionality) into a separate issue while I'll keep. The messy upper layer seems to be of a much narrower sort of value, so I'm postponing it, under this issue, indefinitely.