Parse: Don't consider attributes of broken member declarators

ParseCXXClassMemberDeclaration was trying to use the result of
ActOnCXXMemberDeclarator to attach it to some late parsed attributes.

However when failures arise, we have no decl to attach to which
eventually leads us to a NULL pointer dereference.

While we are here, clean up the code a bit.

Fixes PR16765


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187557 91177308-0d34-0410-b5e6-96231b3b80d8