[yaml2obj][ELF] Make this "type switch" actually readable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184623 91177308-0d34-0410-b5e6-96231b3b80d8
[yaml2obj][ELF] Don't special case writing these.

Just add them to the vector of section headers like the rest of the
section headers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184624 91177308-0d34-0410-b5e6-96231b3b80d8
[yaml2obj][ELF] Narrow parameter.

The full ELFYAML::Section isn't needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184626 91177308-0d34-0410-b5e6-96231b3b80d8
[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