Fix the floating point version of midpoint. It wasn't constexpr, among other things. Add more tests. As a drive-by, the LCD implementation had a class named '__abs' which did a 'absolute value to a common-type' conversion. Rename that to be '__ct_abs'.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363714 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed