Add regression test for SR-10201

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr10201.swift

@@ -0,0 +1,37 @@
+// RUN: %target-swift-frontend -typecheck -verify %s
+
+struct A {
+  typealias Value = Int
+}
+
+protocol B {
+  typealias Value = A.Value
+  typealias T = String
+}
+
+protocol NestedProtocol {
+  typealias _B = B
+}
+
+struct Something: NestedProtocol {
+
+  struct InnerTest: _B {
+    var value: Value = 42
+    var t: T = "wait what?"
+  }
+}
+
+protocol X {}
+
+protocol Y {
+  typealias _X = X
+  var x: _X { get }
+}
+
+struct Struct: Y {
+  var x: _X = __X()
+}
+
+extension Struct {
+  struct __X: _X {}
+}