Add regression test for a fixed crasher

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr4203.swift

@@ -0,0 +1,43 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+public protocol SomeProto {
+    associatedtype ThingType
+	func getThing() -> ThingType
+}
+
+public protocol SpecialThing: RandomAccessCollection {
+}
+
+public protocol Castable {
+	associatedtype Source
+	static func cast(from: Source) -> Self
+}
+
+public struct ThingGetter<P: SomeProto, T> {
+	public let thing: P
+}
+
+extension ThingGetter where P.ThingType: SpecialThing, T: Castable, P.ThingType.Iterator.Element == T.Source {
+
+	public func getView() -> ThingView {
+		return ThingView(thing: thing.getThing())
+	}
+	
+	public struct ThingView: SpecialThing {
+		let thing: P.ThingType
+
+		public typealias Index = P.ThingType.Index
+		public var startIndex: Index    { return thing.startIndex }
+		public var endIndex:   Index    { return thing.startIndex }
+		public var count: Int           { return thing.count }
+		public func index(after i: Index) -> Index {
+			return thing.index(after: i)
+		}
+		public func index(before i: Index) -> Index {
+			return thing.index(before: i)
+		}
+		public subscript(i: Index) -> T {
+			return T.cast(from: thing[i])
+		}
+	}
+}