SFINAE out duration converting constructor if the constructor would otherwise cause a ratio compile-time overflow.  This fixes LWG 2094.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189722 91177308-0d34-0410-b5e6-96231b3b80d8
Forgot to svn add the test for r189722.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189723 91177308-0d34-0410-b5e6-96231b3b80d8