Path: Stop claiming path::const_iterator is bidirectional

path::const_iterator claims that it's a bidirectional iterator, but it
doesn't satisfy all of the contracts for a bidirectional iterator.
For example, n3376 24.2.5 p6 says "If a and b are both dereferenceable,
then a == b if and only if *a and *b are bound to the same object",
but this doesn't work with how we stash and recreate Components.

This means that our use of reverse_iterator on this type is invalid
and leads to many of the valgrind errors we're hitting, as explained
by Tilmann Scheller here:

    http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140728/228654.html

Instead, we admit that path::const_iterator is only an input_iterator,
and implement a second input_iterator for path::reverse_iterator (by
changing const_iterator::operator-- to reverse_iterator::operator++).
All of the uses of this just traverse once over the path in one
direction or the other anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214737 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Path.cpp

@@ -308,7 +308,30 @@ const_iterator &const_iterator::operator++() {
   return *this;
 }
 
-const_iterator &const_iterator::operator--() {
+bool const_iterator::operator==(const const_iterator &RHS) const {
+  return Path.begin() == RHS.Path.begin() && Position == RHS.Position;
+}
+
+ptrdiff_t const_iterator::operator-(const const_iterator &RHS) const {
+  return Position - RHS.Position;
+}
+
+reverse_iterator rbegin(StringRef Path) {
+  reverse_iterator I;
+  I.Path = Path;
+  I.Position = Path.size();
+  return ++I;
+}
+
+reverse_iterator rend(StringRef Path) {
+  reverse_iterator I;
+  I.Path = Path;
+  I.Component = Path.substr(0, 0);
+  I.Position = 0;
+  return I;
+}
+
+reverse_iterator &reverse_iterator::operator++() {
   // If we're at the end and the previous char was a '/', return '.' unless
   // we are the root path.
   size_t root_dir_pos = root_dir_start(Path);
@@ -335,19 +358,11 @@ const_iterator &const_iterator::operator--() {
   return *this;
 }
 
-bool const_iterator::operator==(const const_iterator &RHS) const {
-  return Path.begin() == RHS.Path.begin() &&
+bool reverse_iterator::operator==(const reverse_iterator &RHS) const {
+  return Path.begin() == RHS.Path.begin() && Component == RHS.Component &&
          Position == RHS.Position;
 }
 
-bool const_iterator::operator!=(const const_iterator &RHS) const {
-  return !(*this == RHS);
-}
-
-ptrdiff_t const_iterator::operator-(const const_iterator &RHS) const {
-  return Position - RHS.Position;
-}
-
 const StringRef root_path(StringRef path) {
   const_iterator b = begin(path),
                  pos = b,
@@ -532,7 +547,7 @@ void native(SmallVectorImpl<char> &path) {
 }
 
 const StringRef filename(StringRef path) {
-  return *(--end(path));
+  return *rbegin(path);
 }
 
 const StringRef stem(StringRef path) {