Add now-fixed test case for SR-6746 / rdar://problem/36483751.

(cherry picked from commit 3806d56)

Author: DougGregor

validation-test/compiler_crashers_2_fixed/0140-sr6746.swift

@@ -0,0 +1,8 @@
+// RUN: not %target-swift-frontend %s -typecheck
+struct Foo: Strideable {
+    // typealias Stride = Int
+    let x: Int
+
+    func distance(to other: Foo) -> Foo.Stride { return abs(other.x - x) }
+    func advanced(by n: Foo.Stride) -> Foo { return Foo(x: x + n) }
+}