Tweak query key trait bounds.

Query keys must be stably hashable. Currently this requirement is
expressed as a where-clause on `impl QueryDispatcher for
SemiDynamicQueryDispatcher` and a where-clause on
`create_deferred_query_stack_frame`.

This commit removes those where-clause bounds and adds a single bound to
`QueryCache::Key`, which already has some other bounds. I.e. it
consolidates the bounds. It also gives them a name (`QueryCacheKey`) to
avoid repeating them. There is also a related `Key` trait in
`rustc_middle`; it should probably be merged with `QueryCacheKey` in the
future, but not today.

This cleanup helps with the next two commits, which do bigger
rearrangements, and where the where-clauses caused me some difficulties.

compiler/rustc_query_system/src/lib.rs

@@ -2,6 +2,7 @@
 #![allow(internal_features)]
 #![feature(assert_matches)]
 #![feature(min_specialization)]
+#![feature(trait_alias)]
 // tidy-alphabetical-end
 
 pub mod dep_graph;