docs: LLVM OpenMP build may require clang project even when system clang is installed

## Description

I am trying to build Codon's LLVM fork by following the build-from-source documentation by following official docs ([link](https://docs.exaloop.io/developers/build/))

The documentation currently suggests the LLVMdependency with:

```
git clone --depth 1 -b codon https://github.com/exaloop/llvm-project
cmake -S llvm-project/llvm -B llvm-project/build \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_INCLUDE_TESTS=OFF \
-DLLVM_ENABLE_RTTI=ON \
-DLLVM_ENABLE_ZLIB=OFF \
-DLLVM_ENABLE_ZSTD=OFF \
-DLLVM_ENABLE_PROJECTS="openmp" \
-DLLVM_TARGETS_TO_BUILD=all
```

It also says:
> You can use -DLLVM_ENABLE_PROJECTS="clang;openmp" if you do not have clang installed on your system.

However, on my machine, `clang` is already installed, but the CMake generation step still falls because the OpenMP test targets depend on a CMake target named `clang` , not. just a system-installed `clang`

The error log is below
```
Make Error at cmake/modules/AddLLVM.cmake:2100 (add_dependencies):
The dependency target "clang" of target "check-openmp" does not exist.
Call Stack (most recent call first):
cmake/modules/AddLLVM.cmake:2167 (add_lit_target)
/Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:225 (add_lit_testsuite)
/Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:248 (add_openmp_testsuite)
/Users/rintaro/work/dev/private/llvm-project/openmp/CMakeLists.txt:156 (construct_check_openmp_target)

CMake Error at cmake/modules/AddLLVM.cmake:2100 (add_dependencies):
The dependency target "clang" of target "check-libomp" does not exist.
Call Stack (most recent call first):
cmake/modules/AddLLVM.cmake:2167 (add_lit_target)
/Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:233 (add_lit_testsuite)
/Users/rintaro/work/dev/private/llvm-project/openmp/runtime/test/CMakeLists.txt:44 (add_openmp_testsuite)

CMake Error at cmake/modules/AddLLVM.cmake:2100 (add_dependencies):
The dependency target "clang" of target "check-ompt" does not exist.
Call Stack (most recent call first):
cmake/modules/AddLLVM.cmake:2167 (add_lit_target)
/Users/rintaro/work/dev/private/llvm-project/openmp/cmake/OpenMPTesting.cmake:225 (add_lit_testsuite)
/Users/rintaro/work/dev/private/llvm-project/openmp/runtime/test/CMakeLists.txt:46 (add_openmp_testsuite)
```

After checking the LLVM/OpenMP CMake files, this appears to be a CMake target dependency rather than a lookup for a `clang` executable on `PATH`.

Relevant reference is shown below.
- https://github.com/exaloop/llvm-project/blob/codon/openmp/cmake/OpenMPTesting.cmake#L223-L237
- https://github.com/exaloop/llvm-project/blob/codon/llvm/cmake/modules/AddLLVM.cmake#L2099-L2101

## My Environment

- OS : macOS
- Architecture : arm64