Auto merge of #149709 - Urgau:overhaul-filenames, r=davidtwco

Overhaul filename handling for cross-compiler consistency

This PR overhauls the way we handle filenames in the compiler and `rmeta` in order to achieve achieve cross-compiler consistency (ie. having the same path no matter if the filename was created in the current compiler session or is coming from `rmeta`).

This is required as some parts of the compiler rely on consistent paths for the soundness of generated code (see rust-lang/rust#148328).

In order to achieved consistency multiple steps are being taken by this PR:
 - by making `RealFileName` immutable
 - by only having `SourceMap::to_real_filename` create `RealFileName`
   - currently `RealFileName` can be created from any `Path` and are remapped afterwards, which creates consistency issue
 - by also making `RealFileName` holds it's working directory, embeddable name and the remapped scopes
   - this removes the need for a `Session`, to know the current(!) scopes and cwd, which is invalid as they may not be equal to the scopes used when creating the filename

In order for `SourceMap::to_real_filename` to know which scopes to apply `FilePathMapping` now takes the current remapping scopes to apply, which makes `FileNameDisplayPreference` and company useless and are removed.

This PR is split-up in multiple commits (unfortunately not atomic), but should help review the changes.

Unblocks rust-lang/rust#147611
Fixes rust-lang/rust#148328

Author: bors

src/debuginfo.rs

@@ -297,29 +297,11 @@ impl<'gcc, 'tcx> DebugInfoCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         let pos = span.lo();
         let DebugLoc { file, line, col } = self.lookup_debug_loc(pos);
         match file.name {
-            rustc_span::FileName::Real(ref name) => match *name {
-                rustc_span::RealFileName::LocalPath(ref name) => {
-                    if let Some(name) = name.to_str() {
-                        self.context.new_location(name, line as i32, col as i32)
-                    } else {
-                        Location::null()
-                    }
-                }
-                rustc_span::RealFileName::Remapped {
-                    ref local_path,
-                    virtual_name: ref _unused,
-                } => {
-                    if let Some(name) = local_path.as_ref() {
-                        if let Some(name) = name.to_str() {
-                            self.context.new_location(name, line as i32, col as i32)
-                        } else {
-                            Location::null()
-                        }
-                    } else {
-                        Location::null()
-                    }
-                }
-            },
+            rustc_span::FileName::Real(ref name) => self.context.new_location(
+                name.path(rustc_span::RemapPathScopeComponents::DEBUGINFO).to_string_lossy(),
+                line as i32,
+                col as i32,
+            ),
             _ => Location::null(),
         }
     }