Simplify ThinLTO handling This reduces the amount of complexity around ThinLTO module buffers. It removes duplication between `ModuleBuffer` and `ThinBuffer` (the latter was also used for fat LTO in some cases), clarifies when and where the ThinLTO summary is used (only for `--emit thin-link-bitcode`, ThinLTO performed by rustc rebuilds the summary every time). This also enables a follow up cleanup to reduce code duplication between red, green and imported codegen units. Part of https://github.com/rust-lang/compiler-team/issues/908