Auto merge of #96094 - Elliot-Roberts:fix_doctests, r=compiler-errors

Begin fixing all the broken doctests in `compiler/`

Begins to fix #95994.
All of them pass now but 24 of them I've marked with `ignore HELP (<explanation>)` (asking for help) as I'm unsure how to get them to work / if we should leave them as they are.
There are also a few that I marked `ignore` that could maybe be made to work but seem less important.
Each `ignore` has a rough "reason" for ignoring after it parentheses, with

- `(pseudo-rust)` meaning "mostly rust-like but contains foreign syntax"
- `(illustrative)` a somewhat catchall for either a fragment of rust that doesn't stand on its own (like a lone type), or abbreviated rust with ellipses and undeclared types that would get too cluttered if made compile-worthy.
- `(not-rust)` stuff that isn't rust but benefits from the syntax highlighting, like MIR.
- `(internal)` uses `rustc_*` code which would be difficult to make work with the testing setup.

Those reason notes are a bit inconsistently applied and messy though. If that's important I can go through them again and try a more principled approach. When I run `rg '```ignore \(' .` on the repo, there look to be lots of different conventions other people have used for this sort of thing. I could try unifying them all if that would be helpful.

I'm not sure if there was a better existing way to do this but I wrote my own script to help me run all the doctests and wade through the output. If that would be useful to anyone else, I put it here: https://github.com/Elliot-Roberts/rust_doctest_fixing_tool

compiler/rustc_ast_pretty/src/pp.rs

@@ -68,20 +68,20 @@
 //! will be made to flow subsequent breaks together onto lines. Inconsistent
 //! is the opposite. Inconsistent breaking example would be, say:
 //!
-//! ```
+//! ```ignore (illustrative)
 //! foo(hello, there, good, friends)
 //! ```
 //!
 //! breaking inconsistently to become
 //!
-//! ```
+//! ```ignore (illustrative)
 //! foo(hello, there,
 //!     good, friends);
 //! ```
 //!
 //! whereas a consistent breaking would yield:
 //!
-//! ```
+//! ```ignore (illustrative)
 //! foo(hello,
 //!     there,
 //!     good,
@@ -153,14 +153,14 @@ enum IndentStyle {
     /// Vertically aligned under whatever column this block begins at.
     ///
     ///     fn demo(arg1: usize,
-    ///             arg2: usize);
+    ///             arg2: usize) {}
     Visual,
     /// Indented relative to the indentation level of the previous line.
     ///
     ///     fn demo(
     ///         arg1: usize,
     ///         arg2: usize,
-    ///     );
+    ///     ) {}
     Block { offset: isize },
 }