Auto merge of #12325 - jonas-schievink:clear-check-diags, r=jonas-schievink

fix: Clear `cargo check` diagnostics when flycheck is turned off

Previously stale diagnostics were left over indefinitely when turning off "Check on Save" while diagnostics are present.

crates/rust-analyzer/src/reload.rs

@@ -404,6 +404,7 @@ fn reload_flycheck(&mut self) {
             Some(it) => it,
             None => {
                 self.flycheck = Vec::new();
+                self.diagnostics.clear_check();
                 return;
             }
         };