[yaml2obj][ELF] Remove relationship between ELFState
and ContiguousBlobAccumulator classes. Pass ContiguousBlobAccumulator to
the handleSymtabSectionHeader function directly.

No functional changes.
[yaml2obj][ELF] Move section index to the ELFState class.

No functional changes.
[yaml2obj][ELF] Remove unused typedef.

No functional changes.
[yaml2obj][ELF] Convert some static functions into class members to
reduce number of arguments.

No functional changes.