Invalid `extended_key_value_attribute` error

The error happens too early in the pipeline, and things like doc_comment! { concat!("...", "...") } trigger the error:

        doc_comment! {
            concat!("The smallest value that can be represented by this integer type.
Use [`", stringify!($T), "::MIN", "`](../../std/primitive.", stringify!($T), ".html#associatedconstant.MIN) instead.

# Examples

// unescape this for the testcase, this is just so github doesn't mess up the rendering
\```rust
// deprecated way
let min = std::", stringify!($T), "::MIN;

// intended way
let min = ", stringify!($T), "::MIN;
\```
"),
            #[$attr]
            pub const MIN: $T = $T::MIN;
        }

The error is triggered when parsing, but does not take into account built-in macros that could expand to a string literal (such as concat!()). We need to delay the error emission to after expansion/early nameres

Start by tracing parsing of extended_key_value_attribute through expansion and early name resolution, focusing on built-in macros such as concat!. Reproduce the doc_comment! example, then verify that macros expanding to string literals no longer trigger the error prematurely while invalid attributes still produce an error after expansion.