[libc++] std::abs should not return double

Implement LWG Issue 2735 by adding std::abs
tests for several types and checking their
return value. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369394 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed