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_hir/src/tests.rs

@@ -14,7 +14,7 @@ fn def_path_hash_depends_on_crate_id() {
     // the crate by changing the crate disambiguator (e.g. via bumping the
     // crate's version number).
 
-    create_session_globals_then(Edition::Edition2024, || {
+    create_session_globals_then(Edition::Edition2024, None, || {
         let id0 = StableCrateId::new(Symbol::intern("foo"), false, vec!["1".to_string()], "");
         let id1 = StableCrateId::new(Symbol::intern("foo"), false, vec!["2".to_string()], "");