macro rules shadowing error should emit where the macro being shadowed lives

[gnzlbg]

The macro-expanded `macro_rules!`s may not shadow existing macros should mention where the macro being shadowed was defined.

For example compiling stdsimd I get the following error:

error: `is_x86_feature_detected` is already in scope
   --> crates/stdsimd/src/../../../stdsimd/arch/detect/arch/x86.rs:21:1
    |
21  | / macro_rules! is_x86_feature_detected {
22  | |     ("aes") => {
23  | |         cfg!(target_feature = "aes") || $crate::arch::detect::check_for(
24  | |             $crate::arch::detect::Feature::aes)  };
...   |
165 | |     };
166 | | }
    | |_^
    |
    = note: macro-expanded `macro_rules!`s may not shadow existing macros (see RFC 1560)

error: aborting due to previous error

which tells me that I am defining a macro that is already defined somewhere, but it is not telling me where. That would make this diagnostic much better.