[WIP] [cling] Prototype SYCL support for standalone cling

[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 instead of adding the whole AdaptiveCpp codebase to CLING (like clad) - this is convenient for now to test.
• Clean up CMakeLists:
  • Currently, AdaptiveCpp is installed in a temporary directory to get include directories and acpp binaries for SYCL. Need to fix this.
• Building with ROOT doesn't work
  • 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, with hidden symbols - (need the symbols at runtime, so we cannot hide them), but none lead to fruitful results.
  • Cling builds and works independently, so it can be used in notebooks.
• Add test cases for SYCL
• Investigate DPC++:
  • This seems more promising for integration with ROOT and has the potential to be upstreamed to LLVM in the future. see: https://discourse.llvm.org/t/rfc-add-full-support-for-the-sycl-programming-model/74080
  • Preliminary work trying to get it working with cling patches on top: https://github.com/devajithvs/llvm-project/tree/dev.intel-dpcc

Cling builds and works in standalone mode with adaptiveCpp:
To build (we need the flags -Dadaptivecpp -DLLVM_BUILD_TOOLS=ON -DLLVM_BUILD_LLVM_DYLIB=ON):

cmake -G Ninja  -DCMAKE_BUILD_TYPE=Release -DLLVM_EXTERNAL_PROJECTS="cling" -DLLVM_EXTERNAL_CLING_SOURCE_DIR=../cling -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_TARGETS_TO_BUILD="host;NVPTX" -Dadaptivecpp -DLLVM_BUILD_TOOLS=ON  -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_ENABLE_ASSERTIONS=ON  -DLLVM_LINK_LLVM_DYLIB=ON   ../llvm-project/llvm ```

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

This PR fixes #

[github-actions]

Test Results

0 tests   0 ✅  0s ⏱️
0 suites  0 💤
0 files    0 ❌

Results for commit 15fae9c.