[cmake] Fix LLVM_USE_SPLIT_DWARF

Summary:
- Add `-gsplit-dwarf` as an option instead of a definition.
- Only add that option on compilers supporting dwarf splitting, such as clang
  and gcc.

Reviewers: echristo, pcc

Subscribers: mgorny, aprantl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75328