Provide a default provide* implementation for CodegenBackend
Provide a default provide* implementation for CodegenBackend

Both cg_llvm and cg_clif don't override it. cg_spirv does override it,
so it needs to be preserved.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust