Implement new panic!() behaviour for Rust 2021.

compiler/rustc_span/src/symbol.rs

@@ -398,6 +398,8 @@ symbols! {
         copysignf64,
         core,
         core_intrinsics,
+        core_panic,
+        core_panic_2015_macro,
         core_panic_macro,
         cosf32,
         cosf64,
@@ -794,6 +796,8 @@ symbols! {
         owned_box,
         packed,
         panic,
+        panic_2015,
+        panic_2021,
         panic_abort,
         panic_bounds_check,
         panic_handler,
@@ -1095,6 +1099,8 @@ symbols! {
         staticlib,
         std,
         std_inject,
+        std_panic,
+        std_panic_2015_macro,
         std_panic_macro,
         stmt,
         stmt_expr_attributes,