Add grammar productions for adding 'invariant' to already declared variables.

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@22083 e7fa87d3-cd2b-0410-9028-fcbf551c1848

Test/120.vert

@@ -8,8 +8,9 @@ attribute vec4 attv4;
 uniform sampler2D s2D;
 invariant varying vec2 centTexCoord;
 invariant gl_Position;
-
+centroid gl_Position;
 centroid centroid foo;
+invariant gl_Position, gl_PointSize;
 
 void main()
 {
@@ -22,6 +23,8 @@ void main()
 
     float f[];
     int a = f.length();
+
+    gl_PointSize = 3.8;
 }
 
 uniform float initted = 3.4;