Fix builtin validator for unimplemented opcodes. (#4940)
Fix builtin validator for unimplemented opcodes. (#4940)

This prevents spurious build failures when new opcodeds are added in
SPIRV-Headers.  It moves all the unhandled opcodes to the `default`
case.
KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools