[libc++] Fix broken <random> test

In r369429, I hoisted a floating point computation to a variable in order
to remove a warning. However, it turns out this doesn't play well with
floating point arithmetic. This commit reverts r369429 and instead casts
the result of the floating point computation to remove the warning.

Whether hoisting the computaiton to a variable should give the same
result can be investigated independently.

llvm-svn: 369693