Make ScriptParser::read* functions more functional style.

Previously, many read* functions created new command objects and
add them directly to the top-level data structure. This is not
work for some commands because some commands, such as the assignment,
can appear inside and outside of the output section description.

This patch is to not append objects to the top-level data structure.
Callers are now responsible to do that.

llvm-svn: 277686