Remove QueryContext.

There is a type `QueryCtxt`, which impls the trait `QueryContext`.
Confusingly, there is another type `QueryContext`. The latter is (like
`TyCtxt`) just a pointer to a `GlobalContext`. It's not used much, e.g.
its `impl` block has a single method.

This commit removes `QueryContext`, replacing its use with direct
`GlobalCtxt` use.

compiler/rustc_middle/src/ty/context.rs

@@ -468,6 +468,18 @@ pub struct GlobalCtxt<'tcx> {
     pub(crate) alloc_map: Lock<interpret::AllocMap<'tcx>>,
 }
 
+impl<'tcx> GlobalCtxt<'tcx> {
+    /// Installs `self` in a `TyCtxt` and `ImplicitCtxt` for the duration of
+    /// `f`.
+    pub fn enter<'a: 'tcx, F, R>(&'a self, f: F) -> R
+    where
+        F: FnOnce(TyCtxt<'tcx>) -> R,
+    {
+        let icx = tls::ImplicitCtxt::new(self);
+        tls::enter_context(&icx, || f(icx.tcx))
+    }
+}
+
 impl<'tcx> TyCtxt<'tcx> {
     /// Expects a body and returns its codegen attributes.
     ///