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.

Author: nnethercote

clippy_lints/src/doc/needless_doctest_main.rs

@@ -38,7 +38,7 @@ pub fn check(
     // of all `#[test]` attributes in not ignored code examples
     fn check_code_sample(code: String, edition: Edition, ignore: bool) -> (bool, Vec<Range<usize>>) {
         rustc_driver::catch_fatal_errors(|| {
-            rustc_span::create_session_globals_then(edition, || {
+            rustc_span::create_session_globals_then(edition, None, || {
                 let mut test_attr_spans = vec![];
                 let filename = FileName::anon_source_code(&code);