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

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy 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!
> ```

Sure, thank you. Please see https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/2978

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