Modify APFloat to take a StringRef instead of a c string.

This also adds unit tests to APFloat that mainly tests the
string handling of APFloat, but not much else of it's api.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79210 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed