Sema: Don't assume a nested name specifier holds a type

Sema::PerformObjectMemberConversion assumed that the Qualifier it was
given holds a type. However, the specifier could hold just a namespace.
In this case, we should ignore the qualifier and not attempt to cast to
it.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187715 91177308-0d34-0410-b5e6-96231b3b80d8