Improve Rustdoc's handling of procedural macros

Fixes #58700
Fixes #58696
Fixes #49553
Fixes #52210

This commit removes the special rustdoc handling for proc macros, as we
can now
retrieve their span and attributes just like any other item.

A new command-line option is added to rustdoc: `--crate-type`. This
takes the same options as rustc's `--crate-type` option. However, all
values other than `proc-macro` are treated the same. This allows Rustdoc
to enable 'proc macro mode' when handling a proc macro crate.

In compiletest, a new 'rustdoc-flags' option is added. This allows us to
pass in the '--proc-macro-crate' flag in the absence of Cargo.

I've opened [an additional PR to
Cargo](https://github.com/rust-lang/cargo/pull/7159) to support passing
in this flag.
These two PRS can be merged in any order - the Cargo changes will not
take effect until the 'cargo' submodule is updated in this repository.

src/librustdoc/core.rs

@@ -228,6 +228,7 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
     let RustdocOptions {
         input,
         crate_name,
+        proc_macro_crate,
         error_format,
         libs,
         externs,
@@ -293,11 +294,16 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
     }).collect();
 
     let host_triple = TargetTriple::from_triple(config::host_triple());
+    let crate_types = if proc_macro_crate {
+        vec![config::CrateType::ProcMacro]
+    } else {
+        vec![config::CrateType::Rlib]
+    };
     // plays with error output here!
     let sessopts = config::Options {
         maybe_sysroot,
         search_paths: libs,
-        crate_types: vec![config::CrateType::Rlib],
+        crate_types,
         lint_opts: if !display_warnings {
             lint_opts
         } else {