More vector::iterator debug mode tests.  Run by adding to OPTIONS -D_LIBCPP_DEBUG2=1.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@177897 91177308-0d34-0410-b5e6-96231b3b80d8
Debug mode tests for vector::front and back.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@177904 91177308-0d34-0410-b5e6-96231b3b80d8
Remove some erroneous code I was using to debug debug mode.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@177908 91177308-0d34-0410-b5e6-96231b3b80d8