glsl: Compile error if fs uses gl_FragCoord before first redeclaration

glsl: Compile error if fs uses gl_FragCoord before first redeclaration

Section 4.3.8.1, page 39 of GLSL 1.50 spec says:
  "Within any shader, the first redeclarations of gl_FragCoord
   must appear before any use of gl_FragCoord."

GLSL compiler should generate an error in following case:

vec4 p = gl_FragCoord;
layout(origin_upper_left) in vec4 gl_FragCoord;

void main()
{
}

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>