[clang] Add spirv backend registration guard to syncscope test (PR #215634)

[Justin Bogner via cfe-commits]

bogner wrote:

> We shouldn't have tests that go all the way to a backend in CodeGenHLSL at all. It's probably best to revert the commit that added this test for now to get the build green, and then we should come up with a test that doesn't require the SPIR-V backend here.

Though I suppose it might also be fine to remove this test - making sure `syncscope("workgroup")` is lowered to the right thing in the SPIR-V backend is the job of tests in the SPIR-V backend itself.

https://github.com/llvm/llvm-project/pull/215634