Migrate unstable-in-stable diagnostic

compiler/rustc_error_messages/locales/en-US/const_eval.ftl

@@ -0,0 +1,4 @@
+const-eval-unstable-in-stable =
+    const-stable function cannot use `#[feature({$gate})]`
+    .unstable-sugg = if it is not part of the public API, make this function unstably const
+    .bypass-sugg = otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks

compiler/rustc_error_messages/src/lib.rs

@@ -37,6 +37,7 @@ fluent_messages! {
     parser => "../locales/en-US/parser.ftl",
     privacy => "../locales/en-US/privacy.ftl",
     typeck => "../locales/en-US/typeck.ftl",
+    const_eval => "../locales/en-US/const_eval.ftl",
 }
 
 pub use fluent_generated::{self as fluent, DEFAULT_LOCALE_RESOURCES};