Fix issue #313: Catch internal attempts to modify built-in symbols that don't exist.

Also beefed up support for running compute shaders is #version 420, but this
work is only partially done.

Test/420.comp

@@ -0,0 +1,7 @@
+#version 420
+
+layout(local_size_x = 2) in;  // ERROR, no compute
+
+#extension GL_ARB_compute_shader : enable
+
+layout(local_size_x = 2) in;