Specialize panic_fmt lint for the {core,std}::panic!() macros.

It now only reacts to expansion of those macros, and suggests
inserting `"{}", ` in the right place.

compiler/rustc_span/src/symbol.rs

@@ -393,6 +393,7 @@ symbols! {
         copysignf64,
         core,
         core_intrinsics,
+        core_panic_macro,
         cosf32,
         cosf64,
         crate_id,
@@ -1064,6 +1065,7 @@ symbols! {
         staticlib,
         std,
         std_inject,
+        std_panic_macro,
         stmt,
         stmt_expr_attributes,
         stop_after_dataflow,