Rollup merge of #155036 - bjorn3:lto_refactors16, r=TaKO8Ki

Store a PathBuf rather than SerializedModule for cached modules

In cg_gcc `ModuleBuffer` already only contains a path anyway. And for moving LTO into `-Zlink-only` we will need to serialize `MaybeLtoModules`. By storing a path cached modules we avoid writing them to the disk a second time during serialization of `MaybeLtoModules`.

Some further improvements will require changes to cg_gcc that I would prefer landing in the cg_gcc repo to actually test the LTO changes in CI.

Part of rust-lang/compiler-team#908

Author: JonathanBrouwer

src/back/lto.rs

@@ -144,9 +144,12 @@ fn fat_lto(
     for module in modules {
         match module {
             FatLtoInput::InMemory(m) => in_memory.push(m),
-            FatLtoInput::Serialized { name, buffer } => {
+            FatLtoInput::Serialized { name, bitcode_path } => {
                 info!("pushing serialized module {:?}", name);
-                serialized_modules.push((buffer, CString::new(name).unwrap()));
+                serialized_modules.push((
+                    SerializedModule::from_file(&bitcode_path),
+                    CString::new(name).unwrap(),
+                ));
             }
         }
     }