[SYCL][CUDA] Windows and MSVC support for CUDA backend

[AidanBeltonS]

Patch adds windows support for CUDA backend.

Adds general handling of Windows file paths
Windows support is enabled with remangling of variables from PR #4207 as it fixes mismatch between windows 32-bit longs and default 64-bit long and handles wchar_size.
Adds changes to account for MSVC's default to private classes.
Fixes to unittests for windows.

[bader]

libclc changes look good to me. Just one minor suggestion.

[bader]

@AidanBeltonS, this change introduces a regression: http://ci.llvm.intel.com:8010/#/builders/37/builds/11953/steps/14/logs/FAIL__Clang__cuda-windows_cu. Could you take a look, please?

[AidanBeltonS]

@AidanBeltonS, this change introduces a regression: http://ci.llvm.intel.com:8010/#/builders/37/builds/11953/steps/14/logs/FAIL__Clang__cuda-windows_cu. Could you take a look, please?

Yes, I can try and see what is causing it. Thanks

[AidanBeltonS]

@AidanBeltonS, this change introduces a regression: http://ci.llvm.intel.com:8010/#/builders/37/builds/11953/steps/14/logs/FAIL__Clang__cuda-windows_cu. Could you take a look, please?

Added a fix for the cuda-windows.cu test. Hopefully it should run without error.

[steffenlarsen]

I've built and tested it on a Windows machine using MSVC and only 2 tests in the test suite (intel/llvm-test-suite#383 specifically) fail. I suggest we disregard those 2 tests until both this and intel/llvm-test-suite#383 is merged, then mark them as failing for the CUDA backend on Windows and make issues for each failing test.

Additions LGTM!

[bader]

@smaslov-intel, @intel/llvm-reviewers-runtime, ping.

@AidanBeltonS, could you resolve merge conflict, please?

[AidanBeltonS]

@AidanBeltonS, could you resolve merge conflict, please?

Conflict should be resolved now.

[smaslov-intel]

Please see comments

[steffenlarsen]

A new CUDA backend unittest has been added in sycl\unittests\pi\cuda\test_contexts.cpp. This will need the same treatment as the other unittests.

[smaslov-intel]

LGTM