[libc++] Fix warning C4244 in std/numerics/rand/rand.dist/rand.dist.samp/rand.dist...

[libc++] Fix warning C4244 in std/numerics/rand/rand.dist/rand.dist.samp/rand.dist.samp.discrete/eval.pass.cpp

frederick-vs-ja noticed that https://github.com/microsoft/STL/pull/2976#issuecomment-1201926893
while we are working on updating LLVM submodule for MS STL:

    [...]\std\numerics\rand\rand.dist\rand.dist.samp\rand.dist.samp.discrete\eval.pass.cpp(33): error C2220: the following warning is treated as an error
    [...]\std\numerics\rand\rand.dist\rand.dist.samp\rand.dist.samp.discrete\eval.pass.cpp(287): note: see reference to function template instantiation 'void tests<__int64>(void)' being compiled
    [...]\std\numerics\rand\rand.dist\rand.dist.samp\rand.dist.samp.discrete\eval.pass.cpp(33): warning C4244: 'argument': conversion from '__int64' to 'const unsigned int', possible loss of data

Differential Revision: https://reviews.llvm.org/D130963