Docs are out of date for rustc_codegen_llvm

[camelid]

The docs for rustc_codegen_llvm::base reference functions named codegen_expr and codegen_block, but those functions do not appear anywhere in rustc. It seems the docs for this module have not been updated in two years, so perhaps that is why?

[camelid]

@rustbot modify labels to T-doc C-enhancement

[camelid]

It looks like rustc_codegen_ssa::base has the same docs.

[camelid]

@rustbot modify labels: A-codegen

[camelid]

I finally have enough knowledge to fix these!