Implement --remap-path-prefix

Remove experimental -Zremap-path-prefix-from/to, and replace it with
the stabilized --remap-path-prefix=from=to variant.

This is an implementation for issue of #41555.

src/libsyntax/codemap.rs

@@ -129,7 +129,7 @@ pub struct CodeMap {
     pub(super) files: RefCell<Vec<Rc<FileMap>>>,
     file_loader: Box<FileLoader>,
     // This is used to apply the file path remapping as specified via
-    // -Zremap-path-prefix to all FileMaps allocated within this CodeMap.
+    // --remap-path-prefix to all FileMaps allocated within this CodeMap.
     path_mapping: FilePathMapping,
     stable_id_to_filemap: RefCell<FxHashMap<StableFilemapId, Rc<FileMap>>>,
     /// In case we are in a doctest, replace all file names with the PathBuf,