[NFC] Update llvm version from 19 to 20

[wenju-he]

No description provided.

[wenju-he]

'CodeQL' and 'Out-of-tree build' error:

[ 98%] Linking CXX shared library libopencl-clang.so
/usr/bin/ld: cannot find -lclang-cpp: No such file or directory
/usr/bin/ld: cannot find -lclang-cpp: No such file or directory
collect2: error: ld returned 1 exit status
gmake[2]: *** [CMakeFiles/opencl-clang.dir/build.make:227: libopencl-clang.so.20.0] Error 1

The reason is that libclang-cpp20 isn't correctly installed:

ls -rtl /usr/lib/llvm-20/lib/libclang-cpp*
lrwxrwxrwx 1 root root 43 Sep 22 08:17 /usr/lib/llvm-20/lib/libclang-cpp.so.20.1 -> ../../x86_64-linux-gnu/libclang-cpp.so.20.1
lrwxrwxrwx 1 root root 20 Sep 22 08:17 /usr/lib/llvm-20/lib/libclang-cpp.so -> libclang-cpp.so.20.0
lrwxrwxrwx 1 root root 15 Sep 22 08:17 /usr/lib/llvm-20/lib/libclang-cpp20.so -> libclang-cpp.so

The error is tracked in llvm/llvm-project#109153

[wenju-he]

Will merge after ocl-open-190 is created.