Fix overflow when performing drop check calculations in NLL

Clearing out the infcx's region constraints after processing each type
ends up interacting badly with normalizing associated types. This commit
keeps all region constraints intact until the end of
TypeLivenessGenerator.add_drop_live_constraint, ensuring that normalized
types are able to re-use existing inference variables.

Fixes #47589

src/test/run-pass/nll/issue-47589.rs

@@ -0,0 +1,33 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(nll)]
+
+pub struct DescriptorSet<'a> {
+    pub slots: Vec<AttachInfo<'a, Resources>>
+}
+
+pub trait ResourcesTrait<'r>: Sized {
+    type DescriptorSet: 'r;
+}
+
+pub struct Resources;
+
+impl<'a> ResourcesTrait<'a> for Resources {
+    type DescriptorSet = DescriptorSet<'a>;
+}
+
+pub enum AttachInfo<'a, R: ResourcesTrait<'a>> {
+    NextDescriptorSet(Box<R::DescriptorSet>)
+}
+
+fn main() {
+    let _x = DescriptorSet {slots: Vec::new()};
+}