Stabilize linker-plugin based LTO (aka cross-language LTO)
This PR stabilizes [linker plugin based LTO](https://github.com/rust-lang/rust/issues/49879), also known as "cross-language LTO" because it allows for doing inlining and other optimizations across language boundaries in mixed Rust/C/C++ projects.
As described in the tracking issue, it works by making `rustc` emit LLVM bitcode instead of machine code, the same as `clang` does. A linker with the proper plugin (like LLD) can then run (Thin)LTO across all modules.
The feature has been implemented over a number of pull requests and there are various [codegen](https://github.com/rust-lang/rust/blob/master/src/test/codegen/no-dllimport-w-cross-lang-lto.rs) and [run](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-clang)-[make](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-upstream-rlibs) [tests](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto) that make sure that it keeps working.
It also works for building big projects like [Firefox](https://treeherder.mozilla.org/#/jobs?repo=try&revision=
2ce2d5ddcea6fbff790503eac406954e469b2f5d).
The PR makes the feature available under the `-C linker-plugin-lto` flag. As discussed in the tracking issue it is not cross-language specific and also not LLD specific. `-C linker-plugin-lto` is descriptive of what it does. If someone has a better name, let me know `:)`