Disallow (min) specialization imps with no items

Such implementations are usually mistakes and are not used in the
compiler or standard library (after this commit) so forbid them with
`min_specialization`.

compiler/rustc_hir_analysis/src/errors.rs

@@ -803,6 +803,15 @@ pub(crate) struct ClosureImplicitHrtb {
     pub for_sp: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_empty_specialization)]
+pub(crate) struct EmptySpecialization {
+    #[primary_span]
+    pub span: Span,
+    #[note]
+    pub base_impl_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_const_specialize)]
 pub(crate) struct ConstSpecialize {