Add a couple of crashers that are now fixed

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr12997.swift

@@ -0,0 +1,5 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+func addProperties(b: Int) {
+    guard true else {
+      _ = "`\(b)`."

validation-test/compiler_crashers_2_fixed/sr12999.swift

@@ -0,0 +1,5 @@
+ // RUN: not %target-swift-frontend -typecheck %s
+
+public final class Foo {
+    public var a: String {
+        return "\(backgroundContext)"