[CMake] Make split-file an utility instead of tool

[CMake] Make split-file an utility instead of tool

`split-file` is essentially a development tool used for writing tests. Other related tooling for this purpose such as `FileCheck`, `count`, `not` and more are already created via `add_llvm_utility` instead of `add_llvm_tool`. The later is more meant as LLVM tools used as part of a toolchain (eg. `llvm-ar`), not for tools for the development of LLVM itself.

The main semantic difference this makes is that `split-file` is now built and installed via the `LLVM_INSTALL_UTILS` and `LLVM_BUILD_UTILS` instead of `LLVM_BUILD_TOOLS` and `LLVM_INSTALL_TOOLS` cmake flags. That way one can use `LLVM_BUILD_TOOLS=OFF LLVM_INSTALL_UTILS=ON` and have `split-file` installed along side the other testing tools.

Differential Revision: https://reviews.llvm.org/D130977