glsl: Implement parser support for atomic counters.

glsl: Implement parser support for atomic counters.

v2: Mark atomic counters as read-only variables.  Move offset overlap
    code to the linker.  Use the contains_atomic() convenience method.
v3: Use pointer to integer instead of non-const reference.  Add
    comment so we remember to add a spec quotation from the next GLSL
    release once the issue of atomic counter aggregation within
    structures is clarified.
v4 (idr): Don't use std::map because it's overkill.  Add an assertion
    that ctx->Const.MaxAtomicBufferBindings <= MAX_COMBINED_ATOMIC_BUFFERS.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>