Direct users towards using Rust feature names in CLI

If they are trying to use features rustc doesn't yet know about,
request a feature request.

Additionally, also warn against using feature names without leading `+`
or `-` signs.

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -382,10 +382,7 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     let mut function_features = function_features
         .iter()
         .flat_map(|feat| {
-            llvm_util::to_llvm_feature(cx.tcx.sess, feat)
-                .into_iter()
-                .map(|f| format!("+{}", f))
-                .collect::<Vec<String>>()
+            llvm_util::to_llvm_features(cx.tcx.sess, feat).into_iter().map(|f| format!("+{}", f))
         })
         .chain(codegen_fn_attrs.instruction_set.iter().map(|x| match x {
             InstructionSetAttr::ArmA32 => "-thumb-mode".to_string(),