Updates towards enabling nilability checks

[mppf]

For https://github.com/Cray/chapel-private/issues/305

This PR addresses about the first 100 failures with --no-legacy-nilable-clases. We are working on getting clean testing in that mode in preparation for enabling it by default.

Compiler changes:

• normalizes e.g. _owned(anymanaged MyClass) to _owned(borrowed MyClass)
• improves error checking for PRIM_INIT_VAR and for errorIfNonNilableType when checking for default initialization. In particular, allow some default-init/assign patterns through the use of the flag FLAG_INITIALIZED_LATER.

Module changes:

• use nilable types in SparseBlockDom
• bulkAdd_help uses a locale? in the addOn argument
• Add error for setting a class type to nil via cast or =
• Adjusts makeArrayFromOpaque for nilibility
• Adds a work-around for associative arrays of owned
• Adjusts DistributedBag / DistributedDeque for nilability
• Adjusts the TOML module for nilability and to remove = overloads on classes (see Overloading assignment (=) on class types #5358 (comment) ) and adjusts mason code to use the new TOML set methods
• Adjusts ZMQ.chpl to compile with nilability checking enabled by adjusting the initializers to remove the possibility that any fields store nil.
• Adjusts the conditional in the read function for a channel to make a copy of the argument to read in to - in order avoid default initializing which does not work for non-nilable types.
• Adjusts the Spawn module to always initialize the home field

Additionally adjusts a number of tests to pass with --no-legacy-nilable-classes.

• full local testing
• futures testing

[mppf]

@ben-albrecht - would you please review the TOML/Mason changes?

@lydia-duncan - could you review the makeArrayFromOpaque and ZMQ changes?

@LouisJenkinsCS - FYI about DistributedBag and DistributedDeque changes. Here I took the most obvious approach of using nilable class types MyClass? and then adding ! when calling methods etc. It's likely this could be improved upon in a separate effort (e.g. fields can be non-nilable if they are always initialized).

@benharsh - could you look at the compiler changes and anything else?

Thanks!

[LouisJenkinsCS]

Does the ! stuff add any overhead?

[LouisJenkinsCS]

Honestly if it forces a nil check, is there a way to opt-out of these checks? The bag is meant to be a high-performance data structure, no nil checks are needed, I'd wager.

Also @mppf can you fulfill #13257

[mppf]

Does the ! stuff add any overhead?

These run a nil check that is disabled under --fast.

[ben-albrecht]

TOML/mason changes looked reasonable to me.

[lydia-duncan]

could you review the makeArrayFromOpaque and ZMQ changes?

The makeArrayFromOpaque changes look right. I think the ZMQ changes are good, too, but I'm a little less confident in those since I don't think our testing of it is particularly robust. If it's not too much trouble, would you be up for adding a test about a way it might go wrong based on your changes?