[yaml2obj][XCOFF] add the SectionIndex field for symbol.

Summary: Add the SectionIndex field for symbol.
1: a symbol can reference a section by SectionName or SectionIndex.
2: a symbol can reference a section by both SectionName and SectionIndex.
3: if both Section and SectionIndex are specified, but the two values refer
   to different sections, an error will be reported.
4: an invalid SectionIndex is allowed.
5: if a symbol references a non-existent section by SectionName, an error will be reported.

Reviewed By: jhenderson, Higuoxing

Differential Revision: https://reviews.llvm.org/D109566