[DWARFYAML] Make the include_directories, file_names and opcodes fields of the line...

[DWARFYAML] Make the include_directories, file_names and opcodes fields of the line table optional.

This patch makes the include_directories, file_names and opcodes fields
of the line table optional. This helps us simplify some tests.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D87878