spirv: Rework logging

spirv: Rework logging

This commit reworks the way that logging works in SPIR-V to provide
richer and more detailed logging infrastructure.  This commit contains
several improvements over the old mechanism:

 1) Log messages are now more detailed.  They contain the SPIR-V byte
    offset as well as source language information from OpSource and
    OpLine.

 2) There is now a logging callback mechanism so that errors can get
    propagated to the client through debug callbak extensions.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ian Romanick <idr@freedesktop.org>