This moves the error checking for string tables to getStringTable which returns
an ErrorOr<StringRef>.
This improves error checking, makes it uniform across all string tables and
makes it possible to check them once instead of once per name.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240950 91177308-0d34-0410-b5e6-96231b3b80d8