Add regression test for long-since fixed crasher

Author: slavapestov

test/Generics/issue-55060.swift

@@ -0,0 +1,21 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol E {}
+
+protocol P {
+  associatedtype V: E
+}
+
+protocol Q {
+  associatedtype G: P
+}
+
+protocol R {
+  associatedtype G
+  associatedtype M: Q where M.G == G
+}
+
+struct A<I: E>: P {
+  typealias V = I
+  func f<N: R>(_: N) where N.G == Self {}
+}