Allow configuring the default stage for x.py check

src/bootstrap/config.rs

@@ -377,6 +377,7 @@ struct Build {
     configure_args: Option<Vec<String>>,
     local_rebuild: Option<bool>,
     print_step_timings: Option<bool>,
+    check_stage: Option<u32>,
     doc_stage: Option<u32>,
     build_stage: Option<u32>,
     test_stage: Option<u32>,
@@ -676,6 +677,7 @@ impl Config {
 
         // See https://github.com/rust-lang/compiler-team/issues/326
         config.stage = match config.cmd {
+            Subcommand::Check { .. } => flags.stage.or(build.check_stage).unwrap_or(0),
             Subcommand::Doc { .. } => flags.stage.or(build.doc_stage).unwrap_or(0),
             Subcommand::Build { .. } => flags.stage.or(build.build_stage).unwrap_or(1),
             Subcommand::Test { .. } => flags.stage.or(build.test_stage).unwrap_or(1),
@@ -685,7 +687,6 @@ impl Config {
             // These are all bootstrap tools, which don't depend on the compiler.
             // The stage we pass shouldn't matter, but use 0 just in case.
             Subcommand::Clean { .. }
-            | Subcommand::Check { .. }
             | Subcommand::Clippy { .. }
             | Subcommand::Fix { .. }
             | Subcommand::Run { .. }