[llvm-stress] Ensure that the C++11 random device respects its min/max values (PR32585)

As noted on PR32585, the change in D29780/rL295325 resulted in calls to Rand32() (values 0 -> 0xFFFFFFFF) but the min()/max() operators indicated it would be (0 -> 0x7FFFF).

This patch changes the random operator to call Rand() instead which does respect the 0 -> 0x7FFFF range and asserts that the value is in range as well.

Differential Revision: https://reviews.llvm.org/D34089

llvm-svn: 306281