More automatic extension support

Update grammar table generation:
- Get extensions from instructions, not just operand-kinds
- Don't explicitly list extensions that come from the SPIR-V core
  grammar or from a KHR extended instruction set grammar.

This makes it easier to support new extensions since the recommended
extension strategy is to add instructions to the core grammar file.

Also, test the validator has trivial support for passing through
the extensions SPV_NV_shader_subgroup_partitioned and
SPV_EXT_descriptor_indexing.