Allow Self::Module to be mutated.

`codegen_allocator` and `write_metadata` mutate the underlying LLVM module. As
such, it makes sense for these two functions to receive a mutable reference to
the module (as opposed to an immutable one).

src/librustc_codegen_llvm/lib.rs

@@ -120,11 +120,11 @@ impl ExtraBackendMethods for LlvmCodegenBackend {
     fn write_metadata<'b, 'gcx>(
         &self,
         tcx: TyCtxt<'b, 'gcx, 'gcx>,
-        metadata: &ModuleLlvm
+        metadata: &mut ModuleLlvm
     ) -> EncodedMetadata {
         base::write_metadata(tcx, metadata)
     }
-    fn codegen_allocator(&self, tcx: TyCtxt, mods: &ModuleLlvm, kind: AllocatorKind) {
+    fn codegen_allocator(&self, tcx: TyCtxt, mods: &mut ModuleLlvm, kind: AllocatorKind) {
         unsafe { allocator::codegen(tcx, mods, kind) }
     }
     fn compile_codegen_unit<'a, 'tcx: 'a>(