Fixes for LWG 2598, 2686, 2739, 2742, 2747, and 2759, which were adopted last week in Issaquah

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@286858 91177308-0d34-0410-b5e6-96231b3b80d8
25 files changed