Disable Visual C++ 2013 Debug mode assert on null pointer in some STL algorithms,

such as std::equal on the third argument. This reverts previous workarounds.

Predefining _DEBUG_POINTER_IMPL disables Visual C++ 2013 headers from defining
it to a function performing the null pointer check. In practice, it's not that
bad since any function actually using the nullptr will seg fault. The other
iterator sanity checks remain enabled in the headers.

Reviewed by Aaron Ballmanþ and Duncan P. N. Exon Smith.

llvm-svn: 245711