from user input strings.
Such conversions are more intricate and subtle than they may appear;
it is unlikely I have got it completely right first time. I would
appreciate being informed of any bugs and incorrect roundings you
might discover.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42912 91177308-0d34-0410-b5e6-96231b3b80d8