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_expand/src/mbe/macro_check.rs

@@ -4,7 +4,7 @@
 //!
 //! ## Meta-variables must not be bound twice
 //!
-//! ```
+//! ```compile_fail
 //! macro_rules! foo { ($x:tt $x:tt) => { $x }; }
 //! ```
 //!
@@ -604,9 +604,9 @@ fn check_ops_is_prefix(
 /// Kleene operators of its binder as a prefix.
 ///
 /// Consider $i in the following example:
-///
-///     ( $( $i:ident = $($j:ident),+ );* ) => { $($( $i += $j; )+)* }
-///
+/// ```ignore (illustrative)
+/// ( $( $i:ident = $($j:ident),+ );* ) => { $($( $i += $j; )+)* }
+/// ```
 /// It occurs under the Kleene stack ["*", "+"] and is bound under ["*"] only.
 ///
 /// Arguments:

compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -320,7 +320,8 @@ pub(super) fn count_metavar_decls(matcher: &[TokenTree]) -> usize {
 ///
 /// Then, the tree will have the following shape:
 ///
-/// ```rust
+/// ```ignore (private-internal)
+/// # use NamedMatch::*;
 /// MatchedSeq([
 ///   MatchedSeq([
 ///     MatchedNonterminal(a),