Add regression test for https://bugs.swift.org/browse/SR-13727 / rdar://problem/70298061

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr13727.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+protocol Projection {
+  associatedtype Root
+  associatedtype Path: PartialKeyPath<Root>
+  static var path: Path { get }
+}
+
+struct ProjectKey<Key, Value>: Projection {
+  typealias Root = (Key, Value)
+  typealias Path = KeyPath<(Key, Value), Key>
+  static var path: KeyPath<(Key, Value), Key> { \.0 }
+}