[Chapel Merge] Use skipif/compopts to work around some LLVM issue

Branch: refs/heads/master
Revision: ff56fa6
Author: mppf
Log Message:

Merge pull request #17744 from mppf/skip-llvm-testing-three-extern

Use skipif/compopts to work around some LLVM issues while I investigate

For the sake of nightly testing, uses skipif/compopt to disable 3 tests with LLVM testing.

I'm planning to fix all 3 of these in PR #17409 which gets extern/unions/externUnionNoTypedef.chpl working.

Test change only - not reviewed.

Modified Files:
A test/extern/ferguson/externblock/flexible-array-member.skipif

A test/extern/unions/COMPOPTS

Compare: https://github.com/chapel-lang/chapel/compare/ef05ed5574f8...ff56fa63cb45