[libc++] Avoid implicit conversion warning in a <random> test

By stashing the computation of `E::max() - E::min()` in a variable, we
avoid the warning introduced in r367497. Note that we use `auto` to
avoid having to deduce the type of the computation, which is not a
problem since Clang provides `auto` as an extension even in C++03 (and
we disable warnings related to using C++11 extensions in the test suite).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369429 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed