parser: suggest quotes only for ident attribute values

When encountering code like:

```
#[cfg(key = ["a", "b"])]
fn main() {}
```

the parser incorrectly suggests:

```
error: expected unsuffixed literal, found `[`
 --> ../pg/src/main.rs:1:13
  |
1 | #[cfg(key = ["foo", "bar"])]
  |             ^
  |
help: surround the identifier with quotation marks to parse it as a string
  |
1 | #[cfg(key =" "["foo", "bar"])]
  |            + +

error: aborting due to 1 previous error
```

This commit modifies the parser to check if an attribute value is an
ident before suggesting.

compiler/rustc_parse/src/parser/attr.rs

@@ -412,7 +412,10 @@ impl<'a> Parser<'a> {
         // Check for unquoted idents in meta items, e.g.: #[cfg(key = foo)]
         // `from_expansion()` ensures we don't suggest for cases such as
         // `#[cfg(feature = $expr)]` in macros
-        if self.prev_token == token::Eq && !self.token.span.from_expansion() {
+        if self.token.is_ident()
+            && self.prev_token == token::Eq
+            && !self.token.span.from_expansion()
+        {
             let before = self.token.span.shrink_to_lo();
             while matches!(self.token.kind, token::Ident(..)) {
                 self.bump();