[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

llvm/include/llvm/DebugInfo/DWARF/DWARFDie.h

@@ -404,6 +404,10 @@ public:
       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();

llvm/unittests/DebugInfo/DWARF/DWARFDebugInfoTest.cpp

@@ -1227,6 +1227,10 @@ TEST(DWARFDebugInfo, TestRelations) {
   EXPECT_THAT(std::vector<DWARFDie>(A.rbegin(), A.rend()),
               testing::ElementsAre(D, C, B));
 
+  // Make sure conversion from reverse iterator works as expected.
+  EXPECT_EQ(A.rbegin().base(), A.end());
+  EXPECT_EQ(A.rend().base(), A.begin());
+
   // Make sure iterator is bidirectional.
   {
     auto Begin = A.begin();