Rewrite std::filesystem::path iterators and parser

This patch entirely rewrites the parsing logic for paths. Unlike the previous
implementation this one stores information about the current state; For example
if we are in a trailing separator or a root separator. This avoids the need for
extra lookahead (and extra work) when incrementing or decrementing an iterator.
Roughly this gives us a 15% speedup over the previous implementation.

Unfortunately this implementation is still a lot slower than libstdc++'s.
Because libstdc++ pre-parses and splits the path upon construction their
iterators are trivial to increment/decrement. This makes libc++ lazy parsing
100x slower than libstdc++. However the pre-parsing libstdc++ causes a ton
of extra and unneeded allocations when constructing the string. For example
`path("/foo/bar/")` would require at least 5 allocations with libstdc++
whereas libc++ uses only one. The non-allocating behavior is much preferable
when you consider filesystem usages like 'exists("/foo/bar/")'.

Even then libc++'s path seems to be twice as slow to simply construct compared
to libstdc++. More investigation is needed about this.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@285526 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/experimental/filesystem/class.path/path.itr/iterator_db.pass.cpp

@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+// UNSUPPORTED: libcpp-no-exceptions
+
+// <experimental/filesystem>
+
+// class path
+
+#define _LIBCPP_DEBUG 0
+#define _LIBCPP_ASSERT(cond, msg) ((cond) ? ((void)0) : throw 42)
+
+#include <experimental/filesystem>
+#include <iterator>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+#include "filesystem_test_helper.hpp"
+
+namespace fs = std::experimental::filesystem;
+
+int main() {
+  using namespace fs;
+  // Test incrementing/decrementing a singular iterator
+  {
+    path::iterator singular;
+    try {
+      ++singular;
+      assert(false);
+    } catch (int) {}
+    try {
+      --singular;
+      assert(false);
+    } catch (int) {}
+  }
+  // Test decrementing the begin iterator
+  {
+    path p("foo/bar");
+    auto it = p.begin();
+    try {
+      --it;
+      assert(false);
+    } catch (int) {}
+    ++it;
+    ++it;
+    try {
+      ++it;
+      assert(false);
+    } catch (int) {}
+  }
+  // Test incrementing the end iterator
+  {
+    path p("foo/bar");
+    auto it = p.end();
+    try {
+      ++it;
+      assert(false);
+    } catch (int) {}
+    --it;
+    --it;
+    try {
+      --it;
+      assert(false);
+    } catch (int) {}
+  }
+}