Rollup merge of #77488 - varkor:repr128-incomplete_features, r=jonas-schievink

Mark `repr128` as `incomplete_features`

As mentioned in https://github.com/rust-lang/rust/issues/56071 and noticed in https://github.com/rust-lang/rust/issues/77457, `repr(u128)` and `repr(i128)` do not work properly due to lack of LLVM support. We should thus warn users trying to use the feature that they may encounter ICEs when using it.

Closes https://github.com/rust-lang/rust/issues/77457.

src/tools/clippy/tests/ui/auxiliary/proc_macro_attr.rs

@@ -3,6 +3,7 @@
 
 #![crate_type = "proc-macro"]
 #![feature(repr128, proc_macro_hygiene, proc_macro_quote, box_patterns)]
+#![allow(incomplete_features)]
 #![allow(clippy::useless_conversion)]
 
 extern crate proc_macro;

src/tools/clippy/tests/ui/auxiliary/proc_macro_derive.rs

@@ -3,6 +3,7 @@
 
 #![crate_type = "proc-macro"]
 #![feature(repr128, proc_macro_quote)]
+#![allow(incomplete_features)]
 
 extern crate proc_macro;
 

src/tools/clippy/tests/ui/crashes/auxiliary/proc_macro_crash.rs

@@ -6,6 +6,7 @@
 // contain a proc-macro.
 
 #![feature(repr128)]
+#![allow(incomplete_features)]
 #![crate_type = "proc-macro"]
 
 extern crate proc_macro;