glsl: add always_active_io attribute to ir_variable

glsl: add always_active_io attribute to ir_variable

The value will be set in separate-shader program when an input/output
must remains active. e.g. when deadcode removal isn't allowed because
it will create interface location/name-matching mismatch.

v3:
* Rename the attribute
* Use ir_variable directly instead of ir_variable_refcount_visitor
* Move the foreach IR code in the linker file

v4:
* Fix variable name in assert

v5 (by Timothy Arceri):
* Rename functions and reword comments
* Don't set always active on builtins

Signed-off-by: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>