[DebugInfo] Define base function on DWARFDie reverse iterators

[eush]

Summary

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:

/usr/include/c++/8.2.1/debug/stl_iterator.h:42:35: error: ‘const class std::reverse_iterator<llvm::DWARFDie::iterator>’ has no member named ‘base’
     { return __valid_range(__last.base(), __first.base(), __dist); }
                            ~~~~~~~^~~~

This fixes https://bugs.llvm.org/show_bug.cgi?id=38785.

[dblaikie]

Could you add a unit test for this function while you're adding it?

[eush]

Added a test for the new function.

[JDevlieghere]

Thanks for fixing this and sorry about the oversight. LGTM!

[eush]

Can you commit this change? I don't have commit access.

include/llvm/DebugInfo/DWARF/DWARFDie.h

 
 public:
   reverse_iterator(llvm::DWARFDie::iterator It)
       : Die(It.Die), AtEnd(!It.Die.getPreviousSibling()) {
     if (!AtEnd)
       Die = Die.getPreviousSibling();
   }
 
+  llvm::DWARFDie::iterator base() const {
+    return llvm::DWARFDie::iterator(AtEnd ? Die : Die.getSibling());
+  }
+
   reverse_iterator<llvm::DWARFDie::iterator> &operator++() {
     assert(!AtEnd && "Incrementing rend");
     llvm::DWARFDie D = Die.getPreviousSibling();
     if (D)
       Die = D;
     else
       AtEnd = true;
     return *this;