Fix a couple of bugs in linear_congruential_engine::seed.  Regression test added.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@182421 91177308-0d34-0410-b5e6-96231b3b80d8
Neglected to remove a debugging comment from last commit.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@182422 91177308-0d34-0410-b5e6-96231b3b80d8