[llvm] [CMake] Add root CMakeLists.txt to allow direct CMake execution from root (PR #152753)

[Shaoce SUN via llvm-commits]

sunshaoce wrote:

Thank you all for your feedback. My original intention was simply to add another option to allow building LLVM from the root directory.
I’ll try to address the issues.

https://github.com/llvm/llvm-project/pull/152753