Construct SourceMap at the same time as SessionGlobals.

Currently `SourceMap` is constructed slightly later than
`SessionGlobals`, and inserted. This commit changes things so they are
done at the same time.

Benefits:
- `SessionGlobals::source_map` changes from
  `Lock<Option<Lrc<SourceMap>>>` to `Option<Lrc<SourceMap>>`. It's still
  optional, but mutability isn't required because it's initialized at
  construction.
- `set_source_map` is removed, simplifying `run_compiler`, which is
  good because that's a critical function and it's nice to make it
  simpler.

This requires moving things around a bit, so the necessary inputs are
available when `SessionGlobals` is created, in particular the `loader`
and `hash_kind`, which are no longer computed by `build_session`. These
inputs are captured by the new `SourceMapInputs` type, which is threaded
through various places.

compiler/rustc_log/src/lib.rs

@@ -17,7 +17,7 @@
 //!     rustc_log::init_logger(rustc_log::LoggerConfig::from_env("LOG")).unwrap();
 //!
 //!     let edition = rustc_span::edition::Edition::Edition2021;
-//!     rustc_span::create_session_globals_then(edition, || {
+//!     rustc_span::create_session_globals_then(edition, None, || {
 //!         /* ... */
 //!     });
 //! }