Simplify LazyAttrTokenStream.

This commit does the following.
- Changes it from `Lrc<Box<dyn ToAttrTokenStream>>` to
  `Lrc<LazyAttrTokenStreamInner>`.
- Reworks `LazyAttrTokenStreamImpl` as `LazyAttrTokenStreamInner`, which
  is a two-variant enum.
- Removes the `ToAttrTokenStream` trait and the two impls of it.

The recursion limit must be increased in some crates otherwise rustdoc
aborts.

compiler/rustc_codegen_ssa/src/lib.rs

@@ -14,6 +14,7 @@
 #![feature(string_from_utf8_lossy_owned)]
 #![feature(trait_alias)]
 #![feature(try_blocks)]
+#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 //! This crate contains codegen code that is used by all codegen backends (LLVM and others).