Switch QueryJobId to a single global counter

This replaces the per-shard counters with a single global counter, simplifying
the JobId struct down to just a u64 and removing the need to pipe a DepKind
generic through a bunch of code. The performance implications on non-parallel
compilers are likely minimal (this switches to `Cell<u64>` as the backing
storage over a `u64`, but the latter was already inside a `RefCell` so it's not
really a significance divergence). On parallel compilers, the cost of a single
global u64 counter may be more significant: it adds a serialization point in
theory. On the other hand, we can imagine changing the counter to have a
thread-local component if it becomes worrisome or some similar structure.

The new design is sufficiently simpler that it warrants the potential for slight
changes down the line if/when we get parallel compilation to be more of a
default.

A u64 counter, instead of u32 (the old per-shard width), is chosen to avoid
possibly overflowing it and causing problems; it is effectively impossible that
we would overflow a u64 counter in this context.

compiler/rustc_query_system/src/query/config.rs

@@ -59,7 +59,7 @@ pub trait QueryDescription<CTX: QueryContext>: QueryConfig {
     fn describe(tcx: CTX, key: Self::Key) -> String;
 
     // Don't use this method to access query results, instead use the methods on TyCtxt
-    fn query_state<'a>(tcx: CTX) -> &'a QueryState<CTX::DepKind, Self::Key>
+    fn query_state<'a>(tcx: CTX) -> &'a QueryState<Self::Key>
     where
         CTX: 'a;