Suppress Generator warnings

[samdeane]

Disabled the generator's no support for... warnings by default.

These tend to be a bit noisy:

Generator/default_value: no support for [Variant] = 0
Generator/default_value: no support for [Variant] = 0
Generator/default_value: no support for [Variant] = 0
Generator/default_value: no support for [typedarray::RDPipelineSpecializationConstant] = Array[RDPipelineSpecializationConstant]([])
Generator/default_value: no support for [typedarray::RDPipelineSpecializationConstant] = Array[RDPipelineSpecializationConstant]([])

They can be enabled by defining WARN_MISSING:

• by adding "-Xswiftc" "-D" "-Xswiftc" "WARN_MISSING" to the build arguments when building with SPM
• by adding swiftSettings: [.define("WARN_MISSING")], to the Generator target
• by doing the equivalent in Xcode using the Swift custom flags settings

[migueldeicaza]

Thanks! I had forgotten to remove those when I filed the companion bug:

#585