Features: Add support for C++14 features.

Record the features implemented by GNU 4.9 and Clang 3.4.

Tests/CompileFeatures/cxx_aggregate_default_initializers.cpp

@@ -0,0 +1,9 @@
+
+struct X { int i, j, k = 42; };
+
+int someFunc()
+{
+  X a[] = { 1, 2, 3, 4, 5, 6 };
+  X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };
+  return a[0].k == b[0].k && a[1].k == b[1].k ? 0 : 1;
+}