[LoongArch][Driver] Allow `-gsplit-dwarf` and `-mrelax` to be used together (#175727)
Benefit from https://github.com/llvm/llvm-project/pull/166597 and
https://github.com/llvm/llvm-project/pull/164813, DWARF fission is now
compatible with linker relaxation.
Similar to RISC-V, this commit allows `-gsplit-dwarf` and `-mrelax` to
be used together.
A new test `relax_dwo_ranges.ll` same as RISC-V is also added.