[yaml2obj] - Don't allow setting StOther and Other/Visibility at the same time.

This is a follow up discussed in the comments of D66583.

Currently, if for example, we have both StOther and Other set in YAML document for a symbol,
then yaml2obj reports an "unknown key 'Other'" error.
It happens because 'mapOptional()' is never called for 'Other/Visibility' in this case,
leaving those unhandled.

This message does not describe the reason of the error well. This patch fixes it.

Differential revision: https://reviews.llvm.org/D66642

llvm-svn: 370032