[Chapel Merge] Skip new extern union tests for --llvm compilation

Branch: refs/heads/master
Revision: 78c0843
Author: bradcray
Log Message:

Merge pull request #17363 from bradcray/skip-extern-union-tests-for-llvm

Skip new extern union tests for --llvm compilations

[trivial, not reviewed]

It didn't occur to me to test this before merging, but it seems that
we need to do more work to make the LLVM back-end as union-friendly
as the C back-end. Since we're past feature freeze, skipping these
new tests for --llvm configurations for now.

Modified Files:
A test/extern/unions.skipif

Compare: https://github.com/chapel-lang/chapel/compare/bb2a7770869e...78c0843fb2df