Prevent double panic when handling incremental fingerprint mismatch

When an incremental fingerprint mismatch occurs, we debug-print
our `DepNode` and query result. Unfortunately, the debug printing
process may cause us to run additional queries, which can result
in a re-entrant fingerprint mismatch error.

To avoid a double panic, this commit adds a thread-local variable
to detect re-entrant calls.

compiler/rustc_query_system/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(hash_raw_entry)]
 #![feature(iter_zip)]
 #![feature(min_specialization)]
+#![feature(thread_local_const_init)]
 
 #[macro_use]
 extern crate tracing;