glsl2 main: Use talloc to allocate _mesa_glsl_parse_state

glsl2 main: Use talloc to allocate _mesa_glsl_parse_state

This is a short-lived object. It exists only for the duration of the
compile_shader() function, (as opposed to the shader and whole_program
which live longer).

The state is created with the same talloc parent as the shader, so
that other allocation can be done with talloc_parent(state) as the
owner in order to attach to a long-lived object.