parser: suggest quotes only for ident attribute values

[ohno418]

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.

[rustbot]

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[ohno418]

r? diagnostics

[compiler-errors]

This needs a test

@rustbot author

[ohno418]

Added a test.

@compiler-errors
Could you take another look at this?
r? @compiler-errors

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[bors]

☔ The latest upstream changes (presumably #124972) made this pull request unmergeable. Please resolve the merge conflicts.