[lld] handle typeZeroFill atoms in ELF/Native/YAML

BSS atoms dont take any file space in the Input file. They are associated
with a contentType(typeZeroFill). Similiar zero fill types also exist which
have the same meaning in terms of occupying file space in the Input.

These atoms have to be handled seperately when writing to the
lld's intermediate file or the lld test infrastructure.

Also adds a test.

llvm-svn: 189136