Improve error message when referencing a non-tag type with a tag

Other compilers accept invalid code here that we reject, and we need a
better error message to try to convince users that the code is really
incorrect. Consider:
  class Foo {
    typedef MyIterHelper<Foo> iterator;
    friend class iterator;
  };

Previously our wording was "elaborated type refers to a typedef".
"elaborated type" isn't widely known terminology, so the new diagnostic
says "typedef 'iterator' cannot be referenced with class specifier".

Reviewers: rsmith

Differential Revision: https://reviews.llvm.org/D25216

llvm-svn: 289259