[Chapel Merge] Added C unions to interoperability docs

Branch: refs/heads/master
Revision: 5180381
Author: bradcray
Log Message:

Merge pull request #17310 from souris-dev/cunion-docs

Added C unions to interoperability docs

[contributed by @souris-dev; reviewed by me]

Added examples about C unions in the interoperability docs. Fixes #17294.

[editor's note: This is pretty cool: We've had the ability to interoperate with C unions
for quite some time without necessarily realizing it]

Modified Files:
A test/extern/unions/externUnion.chpl

A test/extern/unions/externUnion.compopts
A test/extern/unions/externUnion.good
A test/extern/unions/externUnionNoTypedef.chpl
A test/extern/unions/externUnionNoTypedef.compopts
A test/extern/unions/externUnionNoTypedef.good
A test/extern/unions/unions.c
A test/extern/unions/unions.h
M doc/rst/language/spec/interoperability.rst

Compare: https://github.com/chapel-lang/chapel/compare/fa817134fb4b...5180381e91ce