Add crashing test case from SR-5546.

Author: DougGregor

validation-test/compiler_crashers_2/0127-sr5546.swift

@@ -0,0 +1,36 @@
+// RUN: not --crash %target-swift-frontend %s -typecheck
+public struct Foo<A, B, C> {}
+
+public protocol P {
+    associatedtype PA
+    associatedtype PB = Never
+    associatedtype PC = Never
+
+    typealias Context = Foo<PA, PB, PC>
+
+    func f1(_ x: Context, _ y: PA)
+    func f2(_ x: Context, _ y: PB)
+    func f3(_ x: Context, _ y: PC)
+    func f4(_ x: Context)
+}
+
+public extension P {
+    public func f1(_ x: Context, _ y: PA) {
+    }
+    public func f2(_ x: Context, _ y: PB) {
+    }
+    public func f3(_ x: Context, _ y: PC) {
+    }
+    public func f4(_ x: Context) {
+    }
+}
+
+public struct S: P {
+    public typealias PA = String
+    public typealias PB = Int
+
+    public func f1(_ x: Context, _ y: PA) {
+    }
+    public func f2(_ x: Context, _ y: PB) {
+    }
+}