rustc: Stabilize -Zrun-dsymutil as -Csplit-debuginfo

This commit adds a new stable codegen option to rustc,
`-Csplit-debuginfo`. The old `-Zrun-dsymutil` flag is deleted and now
subsumed by this stable flag. Additionally `-Zsplit-dwarf` is also
subsumed by this flag but still requires `-Zunstable-options` to
actually activate. The `-Csplit-debuginfo` flag takes one of
three values:

* `off` - This indicates that split-debuginfo from the final artifact is
  not desired. This is not supported on Windows and is the default on
  Unix platforms except macOS. On macOS this means that `dsymutil` is
  not executed.

* `packed` - This means that debuginfo is desired in one location
  separate from the main executable. This is the default on Windows
  (`*.pdb`) and macOS (`*.dSYM`). On other Unix platforms this subsumes
  `-Zsplit-dwarf=single` and produces a `*.dwp` file.

* `unpacked` - This means that debuginfo will be roughly equivalent to
  object files, meaning that it's throughout the build directory
  rather than in one location (often the fastest for local development).
  This is not the default on any platform and is not supported on Windows.

Each target can indicate its own default preference for how debuginfo is
handled. Almost all platforms default to `off` except for Windows and
macOS which default to `packed` for historical reasons.

Some equivalencies for previous unstable flags with the new flags are:

* `-Zrun-dsymutil=yes` -> `-Csplit-debuginfo=packed`
* `-Zrun-dsymutil=no` -> `-Csplit-debuginfo=unpacked`
* `-Zsplit-dwarf=single` -> `-Csplit-debuginfo=packed`
* `-Zsplit-dwarf=split` -> `-Csplit-debuginfo=unpacked`

Note that `-Csplit-debuginfo` still requires `-Zunstable-options` for
non-macOS platforms since split-dwarf support was *just* implemented in
rustc.

There's some more rationale listed on #79361, but the main gist of the
motivation for this commit is that `dsymutil` can take quite a long time
to execute in debug builds and provides little benefit. This means that
incremental compile times appear that much worse on macOS because the
compiler is constantly running `dsymutil` over every single binary it
produces during `cargo build` (even build scripts!). Ideally rustc would
switch to not running `dsymutil` by default, but that's a problem left
to get tackled another day.

Closes #79361

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -282,6 +282,20 @@ pub struct TargetMachineFactoryConfig {
     pub split_dwarf_file: Option<PathBuf>,
 }
 
+impl TargetMachineFactoryConfig {
+    pub fn new(
+        cgcx: &CodegenContext<impl WriteBackendMethods>,
+        module_name: &str,
+    ) -> TargetMachineFactoryConfig {
+        let split_dwarf_file = if cgcx.target_can_use_split_dwarf {
+            cgcx.output_filenames.split_dwarf_filename(cgcx.split_debuginfo, Some(module_name))
+        } else {
+            None
+        };
+        TargetMachineFactoryConfig { split_dwarf_file }
+    }
+}
+
 pub type TargetMachineFactoryFn<B> = Arc<
     dyn Fn(TargetMachineFactoryConfig) -> Result<<B as WriteBackendMethods>::TargetMachine, String>
         + Send
@@ -311,10 +325,11 @@ pub struct CodegenContext<B: WriteBackendMethods> {
     pub tm_factory: TargetMachineFactoryFn<B>,
     pub msvc_imps_needed: bool,
     pub is_pe_coff: bool,
+    pub target_can_use_split_dwarf: bool,
     pub target_pointer_width: u32,
     pub target_arch: String,
     pub debuginfo: config::DebugInfo,
-    pub split_dwarf_kind: config::SplitDwarfKind,
+    pub split_debuginfo: rustc_target::spec::SplitDebuginfo,
 
     // Number of cgus excluding the allocator/metadata modules
     pub total_cgus: usize,
@@ -1035,10 +1050,11 @@ fn start_executing_work<B: ExtraBackendMethods>(
         total_cgus,
         msvc_imps_needed: msvc_imps_needed(tcx),
         is_pe_coff: tcx.sess.target.is_like_windows,
+        target_can_use_split_dwarf: tcx.sess.target_can_use_split_dwarf(),
         target_pointer_width: tcx.sess.target.pointer_width,
         target_arch: tcx.sess.target.arch.clone(),
         debuginfo: tcx.sess.opts.debuginfo,
-        split_dwarf_kind: tcx.sess.opts.debugging_opts.split_dwarf,
+        split_debuginfo: tcx.sess.split_debuginfo(),
     };
 
     // This is the "main loop" of parallel work happening for parallel codegen.