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_codegen_llvm/src/debuginfo/doc.md

@@ -27,9 +27,9 @@ The module is thus driven from an outside client with functions like
 Internally the module will try to reuse already created metadata by
 utilizing a cache. The way to get a shared metadata node when needed is
 thus to just call the corresponding function in this module:
-
-    let file_metadata = file_metadata(cx, file);
-
+```ignore (illustrative)
+let file_metadata = file_metadata(cx, file);
+```
 The function will take care of probing the cache for an existing node for
 that exact file path.
 
@@ -63,7 +63,7 @@ struct List {
 
 will generate the following callstack with a naive DFS algorithm:
 
-```
+```ignore (illustrative)
 describe(t = List)
   describe(t = i32)
   describe(t = Option<Box<List>>)