Add formerly-crashing test case for SR-4786 / rdar://problem/31955862.

(cherry picked from commit 2f00a08)

Author: DougGregor

validation-test/compiler_crashers_2_fixed/0110-sr4786.swift

@@ -0,0 +1,13 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+public protocol _UTFEncoding {
+    associatedtype EncodedScalar where EncodedScalar == Int
+}
+
+public protocol UnicodeEncoding {
+    associatedtype EncodedScalar: BidirectionalCollection
+}
+
+public protocol _UTFParser {
+    associatedtype Encoding: UnicodeEncoding, _UTFEncoding
+}