[DebugInfo] Define base function on DWARFDie reverse iterators

This defines member function base on the specialization of
std::reverse_iterator for DWARFDie::iterator as required by C++
[reverse.iter.conv].

This fixes unit test DWARFDebugInfoTest.cpp under EXPENSIVE_CHECKS which
currently can't be built due to GNU C++ Library calling this member
function in debug mode.

This fixes https://llvm.org/PR38785

Patch by: Eugene Sharygin

Differential revision: https://reviews.llvm.org/D53792

llvm-svn: 345621