Rewrite the untranslatable_diagnostic lint.

Currently it only checks calls to functions marked with
`#[rustc_lint_diagnostics]`. This commit changes it to check calls to
any function with an `impl Into<{D,Subd}iagMessage>` parameter. This
greatly improves its coverage and doesn't rely on people remembering to
add `#[rustc_lint_diagnostics]`.

The commit also adds `#[allow(rustc::untranslatable_diagnostic)`]
attributes to places that need it that are caught by the improved lint.
These places that might be easy to convert to translatable diagnostics.

Finally, it also:
- Expands and corrects some comments.
- Does some minor formatting improvements.
- Adds missing `DecorateLint` cases to
  `tests/ui-fulldeps/internal-lints/diagnostics.rs`.

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -62,6 +62,7 @@ impl<'tcx> NonConstOp<'tcx> for FloatingPointOp {
         }
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
         feature_err(
             &ccx.tcx.sess,
@@ -556,6 +557,7 @@ impl<'tcx> NonConstOp<'tcx> for RawMutPtrDeref {
         Status::Unstable(sym::const_mut_refs)
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
         feature_err(
             &ccx.tcx.sess,
@@ -589,6 +591,7 @@ impl<'tcx> NonConstOp<'tcx> for StaticAccess {
         }
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
         let mut err = feature_err(
             &ccx.tcx.sess,
@@ -632,6 +635,7 @@ pub mod ty {
             }
         }
 
+        #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
             feature_err(
                 &ccx.tcx.sess,