Constant fold int-to-long-double conversions;

Constant fold int-to-long-double conversions;
use APFloat for int-to-float/double; use
round-to-nearest for these (implementation-defined,
seems to match gcc).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42484 91177308-0d34-0410-b5e6-96231b3b80d8