Mark all optimize methods and the codegen method as safe

There is no safety contract and I don't think any of them can actually
cause UB in more ways than passing malicious source code to rustc can.
While LtoModuleCodegen::optimize says that the returned ModuleCodegen
points into the LTO module, the LTO module has already been dropped by
the time this function returns, so if the returned ModuleCodegen indeed
points into the LTO module, we would have seen crashes on every LTO
compilation, which we don't. As such the comment is outdated.

Author: bjorn3

src/lib.rs

@@ -391,7 +391,7 @@ impl WriteBackendMethods for GccCodegenBackend {
         unimplemented!()
     }
 
-    unsafe fn optimize(
+    fn optimize(
         _cgcx: &CodegenContext<Self>,
         _dcx: DiagCtxtHandle<'_>,
         module: &mut ModuleCodegen<Self::Module>,
@@ -409,14 +409,14 @@ impl WriteBackendMethods for GccCodegenBackend {
         Ok(())
     }
 
-    unsafe fn optimize_thin(
+    fn optimize_thin(
         cgcx: &CodegenContext<Self>,
         thin: ThinModule<Self>,
     ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
         back::lto::optimize_thin_module(thin, cgcx)
     }
 
-    unsafe fn codegen(
+    fn codegen(
         cgcx: &CodegenContext<Self>,
         dcx: DiagCtxtHandle<'_>,
         module: ModuleCodegen<Self::Module>,