[yaml2obj][ELF] Allow expressing undefined symbols.

[yaml2obj][ELF] Allow expressing undefined symbols.

Previously we unconditionally enforced that section references in
symbols in the YAML had a name that was a section name present in the
object, and linked the references to that section. Now, permit empty
section names (already the default, if the `Section` key is not
provided) to indicate SHN_UNDEF.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184513 91177308-0d34-0410-b5e6-96231b3b80d8