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 https://github.com/rust-lang/rust/pull/147611
Fixes https://github.com/rust-lang/rust/issues/148328

Cargo.lock

@@ -4630,7 +4630,6 @@ dependencies = [
 name = "rustc_session"
 version = "0.0.0"
 dependencies = [
- "bitflags",
  "getopts",
  "libc",
  "rand 0.9.2",
@@ -4657,6 +4656,7 @@ dependencies = [
 name = "rustc_span"
 version = "0.0.0"
 dependencies = [
+ "bitflags",
  "blake3",
  "derive-where",
  "indexmap",