[CMake] Add option LLVM_FORCE_CREATE_SYMLINKS

On Windows we don't create symlinks for the binaries (clang++, clang-cl)
since the support requires special setup (group policy settings and
you need to know exactly our distribution story). But if you know
about these things and have a controlled environment there is a lot
of storage to be saved, so let's add a manual opt-in for using symlinks
on Windows with LLVM_FORCE_CREATE_SYMLINKS=ON.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D135578