clean up layout error diagnostics

- group the fluent slugs together
- reword (internal-only) "too generic" error to be more in line with
  the other errors

compiler/rustc_middle/src/error.rs

@@ -132,19 +132,19 @@ impl fmt::Debug for CustomSubdiagnostic<'_> {
 
 #[derive(Diagnostic)]
 pub enum LayoutError<'tcx> {
-    #[diag(middle_unknown_layout)]
+    #[diag(middle_layout_unknown)]
     Unknown { ty: Ty<'tcx> },
 
-    #[diag(middle_too_generic)]
+    #[diag(middle_layout_too_generic)]
     TooGeneric { ty: Ty<'tcx> },
 
-    #[diag(middle_values_too_big)]
+    #[diag(middle_layout_size_overflow)]
     Overflow { ty: Ty<'tcx> },
 
-    #[diag(middle_cannot_be_normalized)]
+    #[diag(middle_layout_normalization_failure)]
     NormalizationFailure { ty: Ty<'tcx>, failure_ty: String },
 
-    #[diag(middle_cycle)]
+    #[diag(middle_layout_cycle)]
     Cycle,
 
     #[diag(middle_layout_references_error)]