[ObjectYAML][DWARF] Let the target address size be inferred from FileHeader.

This patch adds a new field `bool Is64bit` in `DWARFYAML::Data` to indicate the address size of target. It's helpful for inferring the `AddrSize` in some DWARF sections.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D81709