rustc_macros: don't limit the -Zmacro-backtrace suggestion to extern macros.

src/test/ui/const-generics/array-impls/core-traits-no-impls-length-33.stderr

@@ -6,6 +6,7 @@ LL |     println!("{:?}", [0_usize; 33]);
    |
    = note: required because of the requirements on the impl of `std::fmt::Debug` for `[usize; 33]`
    = note: required by `std::fmt::Debug::fmt`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: arrays only have std trait implementations for lengths 0..=32
   --> $DIR/core-traits-no-impls-length-33.rs:10:16

src/test/ui/const-generics/broken-mir-2.stderr

@@ -15,6 +15,7 @@ LL | struct S<T: Debug, const N: usize>([T; N]);
    = note: required because of the requirements on the impl of `std::fmt::Debug` for `[T; _]`
    = note: required because of the requirements on the impl of `std::fmt::Debug` for `&[T; _]`
    = note: required for the cast to the object type `dyn std::fmt::Debug`
+   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error
 

src/test/ui/const-generics/derive-debug-array-wrapper.stderr

@@ -15,6 +15,7 @@ LL |     a: [u32; N],
    = note: required because of the requirements on the impl of `std::fmt::Debug` for `[u32; _]`
    = note: required because of the requirements on the impl of `std::fmt::Debug` for `&[u32; _]`
    = note: required for the cast to the object type `dyn std::fmt::Debug`
+   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error