[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.

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