Improve Rustdoc's handling of procedural macros

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.