glsl: Clean-up spurious error message on bad structure definitions
glsl: Clean-up spurious error message on bad structure definitions

Previously a shader like

int X;
struct X { int i; };

void main() { gl_Position = vec4(0.0); }

would generate two error message:

0:2(19): error: struct `X' previously defined
0:2(20): error: incomplete declaration

The first one is the real error, and the second is spurious.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

bkaradzic/glsl-optimizer,mapbox/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,metora/MesaGLSLCompiler,mcanthony/glsl-optimizer,mapbox/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,metora/MesaGLSLCompiler,zeux/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer