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

src/test/ui/if/if-let.stderr

@@ -10,6 +10,7 @@ LL | |     });
    | |_______- in this macro invocation
    |
    = note: `#[warn(irrefutable_let_patterns)]` on by default
+   = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 warning: irrefutable if-let pattern
   --> $DIR/if-let.rs:6:13
@@ -21,6 +22,8 @@ LL | /     bar!(a, 1, {
 LL | |         println!("irrefutable pattern");
 LL | |     });
    | |_______- in this macro invocation
+   |
+   = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 warning: irrefutable if-let pattern
   --> $DIR/if-let.rs:26:5

src/test/ui/if/ifmt-bad-arg.stderr

@@ -300,6 +300,7 @@ LL |     println!("{} {:.*} {}", 1, 3.2, 4);
    |
    = note: expected reference `&usize`
               found reference `&{float}`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0308]: mismatched types
   --> $DIR/ifmt-bad-arg.rs:81:35
@@ -309,6 +310,7 @@ LL |     println!("{} {:07$.*} {}", 1, 3.2, 4);
    |
    = note: expected reference `&usize`
               found reference `&{float}`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 36 previous errors
 

src/test/ui/if/ifmt-bad-format-args.stderr

@@ -3,6 +3,8 @@ error: requires at least a format string argument
    |
 LL |     format_args!();
    |     ^^^^^^^^^^^^^^^
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: format argument must be a string literal
   --> $DIR/ifmt-bad-format-args.rs:3:18

src/test/ui/if/ifmt-unimpl.stderr

@@ -6,6 +6,7 @@ LL |     format!("{:X}", "3");
    |
    = note: required because of the requirements on the impl of `std::fmt::UpperHex` for `&str`
    = note: required by `std::fmt::UpperHex::fmt`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error