[cling] Prototype SYCL support in ROOT

[devajithvs]

This Pull request:

Adds a prototype implementation of SYCL support for standalone Cling. It works for standalone Cling, but is work in progress and is not ready for merge.

Changes or fixes:

Checklist:

• Clone AdaptiveCpp git repo instead of adding the whole AdaptiveCpp codebase to CLING (like clad)
• Clean up CMakeLists:
  • Currently, AdaptiveCpp is installed in a temporary directory to get include directories and acpp binaries for SYCL - now follows what is done for CLAD
• Building with ROOT works now
  • AdaptiveCpp requires LLVM be built with -DLLVM_LINK_LLVM_DYLIB=ON, which unfortunately ROOT doesn't support. Tried to get this working but this causes a cascade of issues. See: Building fails with builtin_llvm=OFF: CommandLine Error: Option 'W' registered more than once! #12156 (comment)
  • Spent time trying various things, building AdaptiveCpp statically is what works. Some patches on top of adaptivecpp will be necessary
  • Cling builds and works independently, so it can be used in notebooks.
• Add test cases for SYCL
• Investigate DPC++:
  • DPC++ does not currently support JIT

To build, you need the flag:
-Dexperimental_adaptivecpp

• tested changes locally
• updated the docs (if necessary)

Root's AdaptiveCpp fork (for static LLVM build and build-time usability - for genvectorx): https://github.com/root-project/AdaptiveCpp

Related: #18461

This PR fixes #

[github-actions]

Test Results

    19 files      19 suites   3d 12h 21m 44s ⏱️
 2 814 tests  2 814 ✅ 0 💤 0 ❌
51 921 runs  51 921 ✅ 0 💤 0 ❌

Results for commit 7a1a305.

♻️ This comment has been updated with latest results.