Track -C link-dead-code during incremental compilation

This option influences monomorphization, which participates in
incremental compilation.

Author: Aaron1011

compiler/rustc_interface/src/tests.rs

@@ -405,7 +405,6 @@ fn test_codegen_options_tracking_hash() {
     untracked!(incremental, Some(String::from("abc")));
     // `link_arg` is omitted because it just forwards to `link_args`.
     untracked!(link_args, vec![String::from("abc"), String::from("def")]);
-    untracked!(link_dead_code, Some(true));
     untracked!(link_self_contained, Some(true));
     untracked!(linker, Some(PathBuf::from("linker")));
     untracked!(linker_flavor, Some(LinkerFlavor::Gcc));
@@ -433,6 +432,7 @@ fn test_codegen_options_tracking_hash() {
     tracked!(force_unwind_tables, Some(true));
     tracked!(inline_threshold, Some(0xf007ba11));
     tracked!(linker_plugin_lto, LinkerPluginLto::LinkerPluginAuto);
+    tracked!(link_dead_code, Some(true));
     tracked!(llvm_args, vec![String::from("1"), String::from("2")]);
     tracked!(lto, LtoCli::Fat);
     tracked!(metadata, vec![String::from("A"), String::from("B")]);

compiler/rustc_session/src/options.rs

@@ -814,7 +814,7 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "a single extra argument to append to the linker invocation (can be used several times)"),
     link_args: Vec<String> = (Vec::new(), parse_list, [UNTRACKED],
         "extra arguments to append to the linker invocation (space separated)"),
-    link_dead_code: Option<bool> = (None, parse_opt_bool, [UNTRACKED],
+    link_dead_code: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "keep dead code at link time (useful for code coverage) (default: no)"),
     link_self_contained: Option<bool> = (None, parse_opt_bool, [UNTRACKED],
         "control whether to link Rust provided C objects/libraries or rely