incr: cache dwarf objects in work products

Cache DWARF objects alongside object files in work products when those
exist so that DWARF object files are available for thorin in packed mode
in incremental scenarios.

Signed-off-by: David Wood <david.wood@huawei.com>

src/test/incremental/split_debuginfo_cached.rs

@@ -0,0 +1,25 @@
+// Check that compiling with packed Split DWARF twice succeeds. This should confirm that DWARF
+// objects are cached as work products and available to the incremental compilation for `thorin` to
+// pack into a DWARF package.
+
+// ignore-tidy-linelength
+// only-x86_64-unknown-linux-gnu
+// revisions:rpass1 rpass2
+
+// [rpass1]compile-flags: -g -Zquery-dep-graph -Zunstable-options -Csplit-debuginfo=packed -Zsplit-dwarf-kind=split
+// [rpass2]compile-flags: -g -Zquery-dep-graph -Zunstable-options -Csplit-debuginfo=packed -Zsplit-dwarf-kind=split
+
+#![feature(rustc_attrs)]
+// For `rpass2`, nothing has changed so everything should re-used.
+#![rustc_partition_reused(module = "split_debuginfo_cached", cfg = "rpass2")]
+#![rustc_partition_reused(module = "split_debuginfo_cached-another_module", cfg = "rpass2")]
+
+mod another_module {
+    pub fn foo() -> &'static str {
+        "hello world"
+    }
+}
+
+pub fn main() {
+    println!("{}", another_module::foo());
+}