Support: Write ScaledNumbers::getRounded()

Start extracting helper functions out of -block-freq's `UnsignedFloat`
into `Support/ScaledNumber.h` with the eventual goal of moving and
renaming the class to `ScaledNumber`.

The bike shed about names is still being painted, but I'm going with
this for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211333 91177308-0d34-0410-b5e6-96231b3b80d8