MCP636: Add simpler and more explicit syntax to check-cfg

This add a new form and deprecated the other ones:
 - cfg(name1, ..., nameN, values("value1", "value2", ... "valueN"))
 - cfg(name1, ..., nameN) or cfg(name1, ..., nameN, values())
 - cfg(any())

It also changes the default exhaustiveness to be enable-by-default in
the presence of any --check-cfg arguments.

compiler/rustc_interface/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(internal_output_capture)]
 #![feature(thread_spawn_unchecked)]
 #![feature(lazy_cell)]
+#![feature(let_chains)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]