Allow attr entries to declare list of alternatives for `List` and `NamedValueStr`

Modify `AttributeTemplate` to support list of alternatives for list and name value attribute styles.

Suggestions now provide more correct suggested code:

```
error[E0805]: malformed `used` attribute input
  --> $DIR/used_with_multi_args.rs:3:1
   |
LL | #[used(compiler, linker)]
   | ^^^^^^------------------^
   |       |
   |       expected a single argument here
   |
help: try changing it to one of the following valid forms of the attribute
   |
LL - #[used(compiler, linker)]
LL + #[used(compiler)]
   |
LL - #[used(compiler, linker)]
LL + #[used(linker)]
   |
LL - #[used(compiler, linker)]
LL + #[used]
   |
```

instead of the prior "masking" of the lack of this feature by suggesting pipe-separated lists:

```
error[E0805]: malformed `used` attribute input
  --> $DIR/used_with_multi_args.rs:3:1
   |
LL | #[used(compiler, linker)]
   | ^^^^^^------------------^
   |       |
   |       expected a single argument here
   |
help: try changing it to one of the following valid forms of the attribute
   |
LL - #[used(compiler, linker)]
LL + #[used(compiler|linker)]
   |
LL - #[used(compiler, linker)]
LL + #[used]
   |
```

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs b/compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs
index b465d2e62ff..1a668b4416f 100644
--- a/compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs
+++ b/compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs
@@ -12,7 +12,7 @@ impl<S: Stage> SingleAttributeParser<S> for RustcLayoutScalarValidRangeStart {
     const PATH: &'static [Symbol] = &[sym::rustc_layout_scalar_valid_range_start];
     const ATTRIBUTE_ORDER: AttributeOrder = AttributeOrder::KeepInnermost;
     const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
-    const TEMPLATE: AttributeTemplate = template!(List: "start");
+    const TEMPLATE: AttributeTemplate = template!(List: &["start"]);
 
     fn convert(cx: &mut AcceptContext<'_, '_, S>, args: &ArgParser<'_>) -> Option<AttributeKind> {
         parse_single_integer(cx, args)
@@ -26,7 +26,7 @@ impl<S: Stage> SingleAttributeParser<S> for RustcLayoutScalarValidRangeEnd {
     const PATH: &'static [Symbol] = &[sym::rustc_layout_scalar_valid_range_end];
     const ATTRIBUTE_ORDER: AttributeOrder = AttributeOrder::KeepInnermost;
     const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
-    const TEMPLATE: AttributeTemplate = template!(List: "end");
+    const TEMPLATE: AttributeTemplate = template!(List: &["end"]);
 
     fn convert(cx: &mut AcceptContext<'_, '_, S>, args: &ArgParser<'_>) -> Option<AttributeKind> {
         parse_single_integer(cx, args)