Rollup merge of #125205 - ChrisDenton:verbatim-include, r=jieyouxu

Fixup Windows verbatim paths when used with the `include!` macro

On Windows, the following code can fail if the `OUT_DIR` environment variable is a [verbatim path](https://doc.rust-lang.org/std/path/enum.Prefix.html) (i.e. begins with `\\?\`):

```rust
include!(concat!(env!("OUT_DIR"), "/src/repro.rs"));
```

This is because verbatim paths treat `/` literally, as if it were just another character in the file name.

The good news is that the standard library already has code to fix this. We can simply use `components` to normalize the path so it works as intended.

compiler/rustc_expand/src/base.rs

@@ -1,5 +1,6 @@
 use std::default::Default;
 use std::iter;
+use std::path::Component::Prefix;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
 
@@ -1293,7 +1294,12 @@ pub fn resolve_path(sess: &Session, path: impl Into<PathBuf>, span: Span) -> PRe
         base_path.push(path);
         Ok(base_path)
     } else {
-        Ok(path)
+        // This ensures that Windows verbatim paths are fixed if mixed path separators are used,
+        // which can happen when `concat!` is used to join paths.
+        match path.components().next() {
+            Some(Prefix(prefix)) if prefix.kind().is_verbatim() => Ok(path.components().collect()),
+            _ => Ok(path),
+        }
     }
 }