[CMake] Add `--gdb-index` option to linker if split dwarf compiling

The option `--gdb-index` should be passed to linker if compiler is invoked
with `-gsplit-dwarf` option. This allows debugger to locate the appropriate
units quickly. See "Invocation" section here: https://gcc.gnu.org/wiki/DebugFission

Suggested by @dblaikie here: https://reviews.llvm.org/D68556#1698442

Differential Revision: https://reviews.llvm.org/D108776