[CUDA] Make min/max shims host+device.

Summary:
Fixes PR37753: min/max can't be called from __host__ __device__
functions in C++14 mode.

Testcase in a separate test-suite commit.

Reviewers: rsmith

Subscribers: sanjoy, lahwaacz, cfe-commits

Differential Revision: https://reviews.llvm.org/D48036

llvm-svn: 336025