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/test.rs

@@ -43,10 +43,16 @@ pub struct TestOptions {
 pub fn run(options: Options) -> i32 {
     let input = config::Input::File(options.input.clone());
 
+    let crate_types = if options.proc_macro_crate {
+        vec![config::CrateType::ProcMacro]
+    } else {
+        vec![config::CrateType::Dylib]
+    };
+
     let sessopts = config::Options {
         maybe_sysroot: options.maybe_sysroot.clone(),
         search_paths: options.libs.clone(),
-        crate_types: vec![config::CrateType::Dylib],
+        crate_types,
         cg: options.codegen_options.clone(),
         externs: options.externs.clone(),
         unstable_features: UnstableFeatures::from_environment(),