Add grammars, C header, and header generator for vendor and KHR extended instruction...

Add grammars, C header, and header generator for vendor and KHR extended instruction sets (#143)

* Add JSON grammars for extened instruction sets

Add AMD extended instruction sets
Add DebugInfo
Add OpenCL.DebugInfo.100

* Add script to generate C headers from extinst grammar

This is cloned then adapted from the same-named script in SPIRV-Tools
(contributed under same authorship but different copyright).

Invoke the script as part of the overall header generation script.

* Add generated C header for extended instruction sets

Add for DebugInfo and OpenCLDebugInfo
Add for AMD vendor extended instruction sets

* Update the README for extinst header generation

* Fix header include guard to match directory structure

* Ensure generated header ends in newline

* Fix typo in file reference

* Fix name of AMD_shader_explicit_vertex_parameter.h

* Avoid duplicate generation

* Split Revision and Version enum values by newlines

Per code review request

* Convert C header generator driver to Python3

* Fix README for Python3 for extinst header generation

* Use 4-space in generated headers, consistently