PR5066: If a declarator cannot have an identifier, and cannot possibly be

PR5066: If a declarator cannot have an identifier, and cannot possibly be
followed by an identifier, then diagnose an identifier as being a bogus part of
the declarator instead of tripping over it. Improves diagnostics for cases like

  std::vector<const int *p> my_vec;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186061 91177308-0d34-0410-b5e6-96231b3b80d8