Return ErrorOr from SymbolRef::getName.

Return ErrorOr from SymbolRef::getName.

This function can really fail since the string table offset can be out of
bounds.

Using ErrorOr makes sure the error is checked.

Hopefully a lot of the boilerplate code in tools/* can go away once we have
a diagnostic manager in Object.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241297 91177308-0d34-0410-b5e6-96231b3b80d8