[clang] [OpenMP] Convert AST dump tests for `variant` clause into codegen tests (PR #204581)

[Johannes Doerfert via cfe-commits]

https://github.com/jdoerfert approved this pull request.

LG, some comments below.
Probably best to move the tests to test/OpenMP now.

https://github.com/llvm/llvm-project/pull/204581