[yaml2obj][ELF] Rename class SectionNameToIdxMap => NameToIdxMap. It can
be used for indexing not only section's names.

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205687 91177308-0d34-0410-b5e6-96231b3b80d8