Add regression test for fixed crasher - rdar://46714549

Author: slavapestov

validation-test/compiler_crashers_2_fixed/rdar46714549.swift

@@ -0,0 +1,6 @@
+// RUN: %target-typecheck-verify-swift
+
+extension Array where Element == String {
+  typealias InternalTypeAlias = Int
+  func doSomething<R>(foo: (InternalTypeAlias) -> R) {}
+}