Qualify panic! as core::panic! in non-built-in core macros

Otherwise code like this

    #![no_implicit_prelude]

    fn main() {
        ::std::todo!();
        ::std::unimplemented!();
    }

will fail to compile, which is unfortunate and presumably unintended.

This changes many invocations of `panic!` in a `macro_rules!` definition
to invocations of `$crate::panic!`, which makes the invocations hygienic.

Note that this does not make the built-in macro `assert!` hygienic.

tests/ui/logic_bug.rs

@@ -1,4 +1,4 @@
-#![allow(unused, clippy::many_single_char_names)]
+#![allow(unused, clippy::many_single_char_names, clippy::diverging_sub_expression)]
 #![warn(clippy::logic_bug)]
 
 fn main() {

tests/ui/nonminimal_bool.rs

@@ -1,4 +1,4 @@
-#![allow(unused, clippy::many_single_char_names)]
+#![allow(unused, clippy::many_single_char_names, clippy::diverging_sub_expression)]
 #![warn(clippy::nonminimal_bool)]
 
 fn main() {

tests/ui/nonminimal_bool_methods.rs

@@ -1,4 +1,4 @@
-#![allow(unused, clippy::many_single_char_names)]
+#![allow(unused, clippy::many_single_char_names, clippy::diverging_sub_expression)]
 #![warn(clippy::nonminimal_bool)]
 
 fn methods_with_negation() {

tests/ui/wildcard_enum_match_arm.fixed

@@ -7,7 +7,7 @@
     dead_code,
     clippy::single_match,
     clippy::wildcard_in_or_patterns,
-    clippy::unnested_or_patterns
+    clippy::unnested_or_patterns, clippy::diverging_sub_expression
 )]
 
 use std::io::ErrorKind;

tests/ui/wildcard_enum_match_arm.rs

@@ -7,7 +7,7 @@
     dead_code,
     clippy::single_match,
     clippy::wildcard_in_or_patterns,
-    clippy::unnested_or_patterns
+    clippy::unnested_or_patterns, clippy::diverging_sub_expression
 )]
 
 use std::io::ErrorKind;