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.
diff --git a/gtests/AST.FromFile.cpp b/gtests/AST.FromFile.cpp
index 3f4819a..08ec919 100644
--- a/gtests/AST.FromFile.cpp
+++ b/gtests/AST.FromFile.cpp
@@ -113,6 +113,7 @@
         "110scope.vert",
         "300scope.vert",
         "400.frag",
+        "420.comp",
         "420.frag",
         "420.vert",
         "420.geom",