[clang][DR] Add test for DR1227 and mark it as complete

DR: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1227

This DR has been implemented in Clang as far back as I could check. This patch simply adds a test and comment to mark it as complete.

Differential Revision: https://reviews.llvm.org/D122682