Fix call of proc_macro::Span::clone on the wrong thread

- This fixes the error message loss (shadowed by an error complaining about `::core` not found) for 2015 edition callers when the callee was using `Error::new_spanned` over the caller's input tokens (since this would then *resolve* `::core` using the caller's edition).

  - an example: wasm-bindgen/wasm-bindgen#3415 (comment) complains about `unresolved import` while pointing to an open parenthesis.


- More generally, it is more correct to only tweak the `located_at` aspect of spans when dealing with a `compile_error!` invocation.

*Resolve* spanned errors at call/mixed site

Mixed site is equivalent to def site for local variables, labels, and
$crate; and equivalent to call site for all other name resolution. Since
it is being used here for a macro invocation which is not one of the
above 3 def site scenarios, directly using call_site is clearer to me.

This reverts commit a091982.

Fixes this error when attempting to build syn with a compiler older
than 1.64:

    error: failed to select a version for the requirement `hashbrown = "^0.14"`
    candidate versions found which didn't match: 0.13.2, 0.13.1, 0.12.3, ...
    location searched: crates.io index
    required by package `indexmap v2.0.0`
        ... which satisfies dependency `indexmap = "^2"` of package `syn-codegen v0.4.0`