The INSTALL_BYPRODUCTS ExternalProject_Add() argument was only added in
CMake 3.26 and the current minimum is 3.20. Work around this by using an
explicit ExternalProject_Add_Step() call for the install step with a
BYPRODUCTS argument. We can't keep using the `install` name here since that
is reserved by the CMake implementation and results in errors when used.
This commit should be reverted once LLVM depends on CMake 3.26.
Pull Request: https://github.com/llvm/llvm-project/pull/115677
5082acce4f