Merge modules and cached_modules for fat LTO

The modules vec can already contain serialized modules and there is no
need to distinguish between cached and non-cached cgus at LTO time.

compiler/rustc_codegen_gcc/src/lib.rs

@@ -359,14 +359,13 @@ impl WriteBackendMethods for GccCodegenBackend {
     fn run_and_optimize_fat_lto(
         cgcx: &CodegenContext<Self>,
         modules: Vec<FatLtoInput<Self>>,
-        cached_modules: Vec<(SerializedModule<Self::ModuleBuffer>, WorkProduct)>,
         diff_fncs: Vec<AutoDiffItem>,
     ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
         if !diff_fncs.is_empty() {
             unimplemented!();
         }
 
-        back::lto::run_fat(cgcx, modules, cached_modules)
+        back::lto::run_fat(cgcx, modules)
     }
 
     fn run_thin_lto(