Bump recursion_limit in rustc_ast_passes

When cfg(parallel_compiler) is enabled, we end up trying to prove
Send/Sync bounds for some deeply nested types (at least when rustdoc is
run).

compiler/rustc_ast_passes/src/lib.rs

@@ -6,6 +6,7 @@
 
 #![feature(bindings_after_at)]
 #![feature(iter_is_partitioned)]
+#![recursion_limit = "256"]
 
 pub mod ast_validation;
 pub mod feature_gate;