[Sema] Don't crash when a field w/ a mem-initializer clashes with a record name

It is possible for a field and a class to have the same name.  In such
cases, performing lookup for the field might return a result set with
more than one entry.  An overzealous assertion fired, causing us to
crash instead of using the non-class lookup result.

This fixes PR28060.

llvm-svn: 272247