Rollup merge of rust-lang#76474 - bjorn3:driver_selected_codegen, r=oli-obk

Add option to pass a custom codegen backend from a driver

This allows the driver to pass information to the codegen backend. For example the headcrab debugger may in the future want to use cg_clif to JIT code to be injected in the debuggee. This would PR make it possible to tell cg_clif which symbol can be found at which address and to tell it to inject the JITed code into the right process.

This PR may also help with rust-lang/miri#1540 by allowing miri to provide a codegen backend that only emits metadata and doesn't perform any codegen.

cc @nbaksalyar (headcrab)
cc @RalfJung (miri)

Author: RalfJung

src/driver.rs

@@ -357,7 +357,7 @@ pub fn main() {
                 args.extend(vec!["--sysroot".into(), sys_root]);
             };
 
-            return rustc_driver::run_compiler(&args, &mut DefaultCallbacks, None, None);
+            return rustc_driver::run_compiler(&args, &mut DefaultCallbacks, None, None, None);
         }
 
         if orig_args.iter().any(|a| a == "--version" || a == "-V") {
@@ -420,6 +420,6 @@ pub fn main() {
         let mut default = DefaultCallbacks;
         let callbacks: &mut (dyn rustc_driver::Callbacks + Send) =
             if clippy_enabled { &mut clippy } else { &mut default };
-        rustc_driver::run_compiler(&args, callbacks, None, None)
+        rustc_driver::run_compiler(&args, callbacks, None, None, None)
     }))
 }