Patch 10 for long double. Doing constants right needs expanding ConstantFP

Patch 10 for long double.  Doing constants right needs expanding ConstantFP
to handle values bigger than double.  If we assume host==target and host
long double works correctly, this is not too bad, but we don't want to
have that limitation longterm.  I could implement accepting double
constants as long double or something like that, which would lead to
incorrect codegen with no errors; the more I think about that the worse
it seems.  Rather than do such a hack that would be backed out later,
I'm settling for giving reasonable error messages, for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40974 91177308-0d34-0410-b5e6-96231b3b80d8