GL_EXT_vulkan_glsl_relaxed - retarget gl_VertexID to gl_VertexIndex

instead of allowing for multiple declarations of the variable in the
resulting SPIR-V, instead use a retargeted mechanism to cause references
to gl_VertexID and gl_InstanceID to use the gl_VertexIndex and
gl_InstanceIndex symbol.

Test/vk.relaxed.stagelink.vert

@@ -25,7 +25,7 @@ vec4 foo() {
 void main() {
 
     vec4 v = foo();
-    v = v + s[gl_VertexID];
-    v.x = v.x - float(gl_InstanceID);
+    v = v + s[gl_VertexID - gl_VertexIndex];
+    v.x = v.x - float(gl_InstanceID - gl_InstanceIndex);
     io = v;
-}
+}