Port #[custom_mir(..)] to the new attribute system

compiler/rustc_span/src/symbol.rs

@@ -446,6 +446,7 @@ symbols! {
         altivec,
         alu32,
         always,
+        analysis,
         and,
         and_then,
         anon,
@@ -587,6 +588,7 @@ symbols! {
         btreemap_contains_key,
         btreemap_insert,
         btreeset_iter,
+        built,
         builtin_syntax,
         c,
         c_dash_variadic,
@@ -851,6 +853,7 @@ symbols! {
         destructuring_assignment,
         diagnostic,
         diagnostic_namespace,
+        dialect,
         direct,
         discriminant_kind,
         discriminant_type,
@@ -1207,6 +1210,7 @@ symbols! {
         infer_static_outlives_requirements,
         inherent_associated_types,
         inherit,
+        initial,
         inlateout,
         inline,
         inline_const,
@@ -1542,6 +1546,7 @@ symbols! {
         opt_out_copy,
         optimize,
         optimize_attribute,
+        optimized,
         optin_builtin_traits,
         option,
         option_env,
@@ -1623,6 +1628,7 @@ symbols! {
         pattern_types,
         permissions_from_mode,
         phantom_data,
+        phase,
         pic,
         pie,
         pin,
@@ -1639,6 +1645,7 @@ symbols! {
         poll,
         poll_next,
         position,
+        post_cleanup: "post-cleanup",
         post_dash_lto: "post-lto",
         postfix_match,
         powerpc_target_feature,
@@ -1802,6 +1809,7 @@ symbols! {
         roundf128,
         rt,
         rtm_target_feature,
+        runtime,
         rust,
         rust_2015,
         rust_2018,