Remove support for -Zcombine-cgu

Nobody seems to actually use this, while still adding some extra
complexity to the already rather complex codegen coordinator code.
It is also not supported by any backend other than the LLVM backend.

compiler/rustc_codegen_gcc/src/back/write.rs

@@ -4,7 +4,6 @@ use gccjit::{Context, OutputKind};
 use rustc_codegen_ssa::back::link::ensure_removed;
 use rustc_codegen_ssa::back::write::{BitcodeSection, CodegenContext, EmitObj, ModuleConfig};
 use rustc_codegen_ssa::{CompiledModule, ModuleCodegen};
-use rustc_errors::DiagCtxtHandle;
 use rustc_fs_util::link_or_copy;
 use rustc_session::config::OutputType;
 use rustc_span::fatal_error::FatalError;
@@ -258,14 +257,6 @@ pub(crate) fn codegen(
     ))
 }
 
-pub(crate) fn link(
-    _cgcx: &CodegenContext<GccCodegenBackend>,
-    _dcx: DiagCtxtHandle<'_>,
-    mut _modules: Vec<ModuleCodegen<GccContext>>,
-) -> Result<ModuleCodegen<GccContext>, FatalError> {
-    unimplemented!();
-}
-
 pub(crate) fn save_temp_bitcode(
     cgcx: &CodegenContext<GccCodegenBackend>,
     _module: &ModuleCodegen<GccContext>,

compiler/rustc_codegen_gcc/src/lib.rs

@@ -426,14 +426,6 @@ impl WriteBackendMethods for GccCodegenBackend {
     fn serialize_module(_module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer) {
         unimplemented!();
     }
-
-    fn run_link(
-        cgcx: &CodegenContext<Self>,
-        dcx: DiagCtxtHandle<'_>,
-        modules: Vec<ModuleCodegen<Self::Module>>,
-    ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
-        back::write::link(cgcx, dcx, modules)
-    }
 }
 
 /// This is the entrypoint for a hot plugged rustc_codegen_gccjit