[yaml2obj][ELF] Make symbol table top-level key.

Although in reality the symbol table in ELF resides in a section, the
standard requires that there be no more than one SHT_SYMTAB. To enforce
this constraint, it is cleaner to group all the symbols under a
top-level `Symbols` key on the object file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184627 91177308-0d34-0410-b5e6-96231b3b80d8