[DWARFYAML] Refactor emitDebugInfo() to make the length be inferred.

This patch refactors `emitDebugInfo()` to make the length field be
inferred from its content. Besides, the `Visitor` class is removed in
this patch. The original `Visitor` class helps us determine an
appropriate length and emit the .debug_info section. These two
processes can be merged into one process. Besides, the length field
should be inferred when it's missing rather than when it's zero.

Reviewed By: jhenderson, labath

Differential Revision: https://reviews.llvm.org/D84008