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]
   |
```

0 files changed, 0 insertions, 0 deletions