[clang] Add test for CWG600

P1787: //CWG600 is resolved by explaining that accessibility affects naming a member in the sense of the ODR.//
Wording: see changes to [class.access] p1 and p4.
Additional references: [[ http://eel.is/c++draft/basic.def.odr#8.sentence-2 | basic.def.odr/8 ]]: //A function is odr-used if it is named by a potentially-evaluated expression or conversion.//

Reviewed By: #clang-language-wg, aaron.ballman

Differential Revision: https://reviews.llvm.org/D139173