st/mesa/i965: create link status enum

For the on-disk shader cache we want to be able to differentiate
between a program that was linked and one that was loaded from cache.

V2:
 - don't return the new enum directly to the application when queried,
   instead return GL_TRUE or GL_FALSE as required. Fixes google-chrome
   corruptions when using cache.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/compiler/glsl/linker.cpp

@@ -421,7 +421,7 @@ linker_error(gl_shader_program *prog, const char *fmt, ...)
    ralloc_vasprintf_append(&prog->data->InfoLog, fmt, ap);
    va_end(ap);
 
-   prog->data->LinkStatus = false;
+   prog->data->LinkStatus = linking_failure;
 }
 
 
@@ -2190,7 +2190,7 @@ link_intrastage_shaders(void *mem_ctx,
                              _mesa_shader_stage_to_program(shader_list[0]->Stage),
                              prog->Name, false);
    if (!gl_prog) {
-      prog->data->LinkStatus = false;
+      prog->data->LinkStatus = linking_failure;
       _mesa_delete_linked_shader(ctx, linked);
       return NULL;
    }
@@ -4629,7 +4629,7 @@ linker_optimisation_loop(struct gl_context *ctx, exec_list *ir,
 void
 link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
 {
-   prog->data->LinkStatus = true; /* All error paths will set this to false */
+   prog->data->LinkStatus = linking_success; /* All error paths will set this to false */
    prog->data->Validated = false;
 
    /* Section 7.3 (Program Objects) of the OpenGL 4.5 Core Profile spec says:

src/compiler/glsl/standalone.cpp

@@ -509,7 +509,7 @@ standalone_compile_shader(const struct standalone_options *_options,
       } else {
          const gl_shader_stage stage = whole_program->Shaders[0]->Stage;
 
-         whole_program->data->LinkStatus = GL_TRUE;
+         whole_program->data->LinkStatus = linking_success;
          whole_program->_LinkedShaders[stage] =
             link_intrastage_shaders(whole_program /* mem_ctx */,
                                     ctx,