Revert "Extension: Mark the default constructor of chrono::duration as conditionally noexcept"; this breaks the gcc5 bot for C++11

This reverts commit c8ca15c95c4c0d6d1356500d5fe49a319ea4ca01.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@372546 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed