Another vector debug mode test, and a static test on Allocator::value_type.  This partially addresses http://llvm.org/bugs/show_bug.cgi?id=15576.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178064 91177308-0d34-0410-b5e6-96231b3b80d8