Document some safety constraints and use more safe wrappers

compiler/rustc_codegen_llvm/src/declare.rs

@@ -235,7 +235,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
     /// name.
     pub(crate) fn get_defined_value(&self, name: &str) -> Option<&'ll Value> {
         self.get_declared_value(name).and_then(|val| {
-            let declaration = unsafe { llvm::LLVMIsDeclaration(val) != 0 };
+            let declaration = llvm::is_declaration(val);
             if !declaration { Some(val) } else { None }
         })
     }