Rollup merge of #44694 - tommyip:make_clean, r=Mark-Simulacrum

Add --all flag to ./x.py clean

This make `clean` removes the LLVM and download cache directory as well.

Fixes #44214.

r? @Mark-Simulacrum

src/bootstrap/lib.rs

@@ -345,8 +345,8 @@ impl Build {
             job::setup(self);
         }
 
-        if let Subcommand::Clean = self.config.cmd {
-            return clean::clean(self);
+        if let Subcommand::Clean { all } = self.config.cmd {
+            return clean::clean(self, all);
         }
 
         self.verbose("finding compilers");