Patch by Howard. First part of fix for PR18218; add type traits needed to do the right thing. Fix the problems in PR18218 for isnan and pow - they also need to be applied to the other functions in <cmath>. Also, a drive-by fix for the test - now actually calls test_abs()

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@198431 91177308-0d34-0410-b5e6-96231b3b80d8