[TypeChecker] NFC: Add a test-case for rdar://problem/50869732

(cherry picked from commit 4fdc9ff)

Author: xedin

validation-test/Sema/type_checker_crashers_fixed/rdar50869732.swift

@@ -0,0 +1,34 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {
+  associatedtype T : P
+}
+
+struct Generic<T> {
+  init(_ value: T) {}
+}
+
+@_functionBuilder
+struct Builder {
+  static func buildBlock<C0, C1>(_ c0: C0, _ c1: C1)
+           -> Generic<(C0, C1)> where C0 : P, C1 : P {
+    return Generic((c0, c1))
+  }
+}
+
+struct G<C> {
+  init(@Builder _: () -> C) {}
+}
+
+struct Empty {
+  init() {}
+}
+
+struct Test<T> where T : P {
+  init(@Builder _: () -> T) {}
+}
+
+let x = G {
+  Empty()
+  Test { <#code#> } // expected-error {{editor placeholder in source file}}
+}