Auto merge of #153968 - jyn514:jyn/linker-warn-by-default, r=mati865

linker-messages is warn-by-default again

cc rust-lang/rust#136096 

I ended up keeping it a lint and adding an option for lints to ignore `-Dwarnings` (there was already a lint that did that actually, it was just hard-coded in rustc_middle instead of in rustc_lint_defs like I'd expect). This allows people to actually see the warnings without them failing the build in CI.

Author: bors