[llvm] [SPIR-V] Change a way SPIR-V Backend API works with user facing options (PR #124745)

[Sven van Haastregt via llvm-commits]

svenvh wrote:

@VyacheslavLevytskyy are you planning to update the API usage in the SPIRV-LLVM-Translator too?  I believe this commit broke its `spirvbackend-usage.ll` test:

```
Fails to save LLVM as SPIR-V: Invalid optimization level!
```

https://github.com/llvm/llvm-project/pull/124745