Move debuginfo docs from doc.rs module to doc.md file

And use `#[doc = include_str!("doc.md")]` in `mod.rs` so the docs are
rendered as if they were inline in the root module.

compiler/rustc_codegen_llvm/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(bool_to_option)]
 #![feature(const_cstr_unchecked)]
 #![feature(crate_visibility_modifier)]
+#![feature(extended_key_value_attributes)]
 #![feature(extern_types)]
 #![feature(in_band_lifetimes)]
 #![feature(nll)]