Add tracking for extension based warnings

Using '#extension foo: warn' instructs the compiler to generate a
warning when some feature of the extension 'foo' is used.  This patch
adds some infrastructure needed to support that for variables.

Similar changes will be needed for types and built-in functions.

ir.h

@@ -156,6 +156,11 @@ public:
     */
    unsigned array_lvalue:1;
 
+   /**
+    * Emit a warning if this variable is accessed.
+    */
+   const char *warn_extension;
+
    /**
     * Value assigned in the initializer of a variable declared "const"
     */