UPC++ 2020.3.0 update (#15623)

## Summary

This PR updates and improves the Spack package for [UPC++](https://upcxx.lbl.gov).
I'm an LBL employee and developer on the UPC++ team, as well as the maintainer of this Spack package.

### Key Improvements:
* Adding new 2020.3.0 release and support for use of develop/master branches
    - Our build infrastructure underwent a major change in this release, switching from a hand-rolled Python2 script to a bash-based autoconf work-alike. 
    - The new build system is NOT using autotools (nor does it support some of the more esoteric autoconf options), but the user interface for common builds is similar.
* Add explicit support for an MPI optional dependency
    - New `mpi` variant enables use of the MPI-based spawner (most relevant on loosely coupled clusters), and the (unofficial) mpi-conduit backend
    - This variant is OFF by default, since UPC++ works fine without MPI on many systems, increasing the likelihood first-time Spack users get a working build without needing to correctly setup MPI
* Add support for post-install testing using the test support deployed in the new build infrastructure
* Fix or workaround a few bugs observed during testing 

### Status

The new package has been validated with a variety of specs across over seven different systems, including: NERSC cori, ALCF Theta, OLCF Summit, an in-house Linux cluster, and macOS laptops (Mojave and Catalina).  

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