Add -Z llvm_module_flag Allow adding values to the `!llvm.module.flags` metadata for a generated module. The syntax is `-Z llvm_module_flag=<name>:<type>:<value>:<behavior>` Currently only u32 values are supported but the type is required to be specified for forward compatibility. The `behavior` element must match one of the named LLVM metadata behaviors.viors. This flag is expected to be perma-unstable.