[Driver] -gsplit-dwarf: Produce .dwo regardless of -gN for IR input

This generalizes D94647 to IR input, as suggested by @tejohnson.
Ideally the driver should just forward split dwarf options, but doing this currently will cause `clang -gsplit-dwarf -c a.c` to create a .dwo with just `.strtab`.

Reviewed By: dblaikie, tejohnson

Differential Revision: https://reviews.llvm.org/D94655