Simplify grammar for identifier_list

[gnl21]

An identifier list was defined as one or more identifiers preceded by a comma, so an additional rule was needed for a single identifier. Change the identifier_list to be what would normally be expected, a comma-separated list of one or more identifiers, and remove the additional rule.

[gnl21]

@dj2 this was my first grammar change that was not taken up. The grammar is full of things like this that I consider to be bizarre and unhelpful definitions of commonplace things.

[gnl21]

Update to glslang to use the simpler grammar here: https://github.com/KhronosGroup/glslang/pull/3772