Don't require each rustc_interface tool to opt-in to parallel_rustc support

Previously, forgetting to call `interface::set_thread_safe_mode` would cause the following ICE:
```
thread 'rustc' panicked at 'uninitialized dyn_thread_safe mode!', /rustc/dfe0683138de0959b6ab6a039b54d9347f6a6355/compiler/rustc_data_structures/src/sync.rs:74:18
```

This calls `set_thread_safe_mode` in `interface::run_compiler` to avoid requiring it in the caller.

Fixes `tests/run-make-fulldeps/issue-19371` when parallel-compiler is enabled.

compiler/rustc_driver_impl/src/lib.rs

@@ -279,9 +279,6 @@ fn run_compiler(
 
     let sopts = config::build_session_options(&mut early_error_handler, &matches);
 
-    // Set parallel mode before thread pool creation, which will create `Lock`s.
-    interface::set_thread_safe_mode(&sopts.unstable_opts);
-
     if let Some(ref code) = matches.opt_str("explain") {
         handle_explain(&early_error_handler, diagnostics_registry(), code, sopts.color);
         return Ok(());