[CUDA] Make __clang_cuda_cmath.h compatible with libc++.

Summary:
We need to add a bunch more "using"s, which weren't necessary with
libstdc++.

Once this is in I can check in a test to the test-suite.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24588

llvm-svn: 281544