Add --print=split-debuginfo

This option prints all supported values for -Csplit-debuginfo=.., i.e.
only stable ones on stable/beta and all of them on nightly/dev.

compiler/rustc_driver/src/lib.rs

@@ -736,6 +736,17 @@ fn print_crate_info(
             // Any output here interferes with Cargo's parsing of other printed output
             NativeStaticLibs => {}
             LinkArgs => {}
+            SplitDebuginfo => {
+                use rustc_target::spec::SplitDebuginfo::{Off, Packed, Unpacked};
+
+                for split in &[Off, Packed, Unpacked] {
+                    let stable = sess.target.options.supported_split_debuginfo.contains(split);
+                    let unstable_ok = sess.unstable_options();
+                    if stable || unstable_ok {
+                        println!("{}", split);
+                    }
+                }
+            }
         }
     }
     Compilation::Stop

compiler/rustc_session/src/config.rs

@@ -548,6 +548,7 @@ pub enum PrintRequest {
     NativeStaticLibs,
     StackProtectorStrategies,
     LinkArgs,
+    SplitDebuginfo,
 }
 
 pub enum Input {
@@ -1806,6 +1807,7 @@ fn collect_print_requests(
         ("stack-protector-strategies", PrintRequest::StackProtectorStrategies),
         ("target-spec-json", PrintRequest::TargetSpec),
         ("link-args", PrintRequest::LinkArgs),
+        ("split-debuginfo", PrintRequest::SplitDebuginfo),
     ];
 
     prints.extend(matches.opt_strs("print").into_iter().map(|req| {

src/test/run-make/valid-print-requests/valid-print-requests.stderr

@@ -1,2 +1,2 @@
-error: unknown print request `uwu`. Valid print requests are: `crate-name`, `file-names`, `sysroot`, `target-libdir`, `cfg`, `calling-conventions`, `target-list`, `target-cpus`, `target-features`, `relocation-models`, `code-models`, `tls-models`, `native-static-libs`, `stack-protector-strategies`, `target-spec-json`, `link-args`
+error: unknown print request `uwu`. Valid print requests are: `crate-name`, `file-names`, `sysroot`, `target-libdir`, `cfg`, `calling-conventions`, `target-list`, `target-cpus`, `target-features`, `relocation-models`, `code-models`, `tls-models`, `native-static-libs`, `stack-protector-strategies`, `target-spec-json`, `link-args`, `split-debuginfo`