[SYCL] Remove re-flower pass

[bader]

Current approach with translation to SPIR-V uses re-flower pass. See this section for more details: https://github.com/intel/llvm/blob/sycl/sycl/doc/SYCL_compiler_and_runtime_design.md#integration-with-spir-v-format
To simplify SYCL compiler implementation and upstreaming work we should investigate if it's possible to translate SYCL to SPIR-V w/o this pass.

According to my understanding the main problem this pass solves is name conflicts.
For instance, type names defined for SPIR-V can conflict with user types - e.g. OpTypeEvent.

SPIR-V in LLVM IR format description: https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/master/docs/SPIRVRepresentationInLLVM.rst

[Naghasan]

Those types could be defined as a predefined types in Sema in a non conflicting way.

i.e. OpTypeEvent could become in the header file __spirv_OpTypeEvent. In clang's Sema, the init would predefine this type __spirv_OpTypeEvent when SYCL is enable to some internal type (maybe reuse the clang's OpenCL event type ?) which can then be lowered in a similar way as it is done for OpenCL in CodeGen.

One issue here: there is a conflicts between the mangling for SPIR and the SPIR-V convention.

[bader]

@Naghasan, we are working on a bit different approach for SPIR-V types. Specifically for OpTypeEvent, we are trying to re-use OpenCL event_t in SYCL.
You can take the idea how it will be done from #167.
@Fznamznon, could you create a PR with your current state or update #167, please?

[Naghasan]

That's very similar to what I had in mind, the problem will just be the mangler in the end (i.e. having a mangling that make sense for both OpenCL C and the SPIR-V translator).

[Fznamznon]

@bader , @Naghasan , I've created PR with my changes - #171
I'm trying to re-use OpenCL event and sampler types -076d7a1