Patch by Howard. First part of fix for PR18218; add type traits needed to do...

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()

llvm-svn: 198431