[ADT] Add basic operator overloads for arithmetic to APFloat to make
code using it more readable.

Also add a copySign static function that works more like the standard
function by accepting the value and sign-carying value as arguments.

No interesting logic here, but tests added to cover the basic API
additions and make sure they do something plausible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219453 91177308-0d34-0410-b5e6-96231b3b80d8