Add attribute noreturn to functions that throw

Reviewers: mclow.lists, EricWF, howard.hinnant, sebpop
Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D21232

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@279903 91177308-0d34-0410-b5e6-96231b3b80d8