getTrueType minor fix

In getTrueType, the T_DESCRIPT case does not behave quite as one would

The original code is this:

        case T_DESCRIPT:
           return this->maskDescriptors (mask);

Please replace it with this code:

       case T_DESCRIPT:
	    SgType *masked = this->maskDescriptors (mask);

	    /* maskDescriptors returns a TrueType, but it
	       does not follow typedefs, so we might still
	       need to do that. */

	    if ( masked->variant() == T_DESCRIPT
		 || ( ! follow_typedefs ) )
		 /* Did not get rid of all of the descriptors,
		    or not following typdefs. */
		 return masked;
		 /* Got rid of all descriptors and are following typedefs. */
		 return masked->getTrueType(mask, follow_typedefs);