[DWARFYAML][debug_info] Pull out dwarf::FormParams from DWARFYAML::Unit.

Unit.Format, Unit.Version and Unit.AddrSize are replaced with
dwarf::FormParams in D84496 to get rid of unnecessary functions
getOffsetSize() and getRefSize(). However, that change makes it
difficult to make AddrSize optional (Optional<uint8_t>). This change
pulls out dwarf::FormParams from DWARFYAML::Unit and use it as a helper
struct in DWARFYAML::emitDebugInfo().

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D85296