Implement filesystem NB comments, relative paths, and related issues.

This is a fairly large patch that implements all of the filesystem NB comments
and the relative paths changes (ex. adding weakly_canonical). These issues
and papers are all interrelated so their implementation couldn't be split up
nicely.

This patch upgrades <experimental/filesystem> to match the C++17 spec and not
the published experimental TS spec. Some of the changes in this patch are both
API and ABI breaking, however libc++ makes no guarantee about stability for
experimental implementations.

The major changes in this patch are:

* Implement NB comments for filesystem (P0492R2), including:
  * Implement `perm_options` enum as part of NB comments, and update the
    `permissions` function to match.
  * Implement changes to `remove_filename` and `replace_filename`
  * Implement changes to `path::stem()` and `path::extension()` which support
    splitting examples like `.profile`.
  * Change path iteration to return an empty path instead of '.' for trailing
    separators.
  * Change `operator/=` to handle absolute paths on the RHS.
  * Change `absolute` to no longer accept a current path argument.

* Implement relative paths according to NB comments (P0219r1)

* Combine `path.cpp` and `operations.cpp` since some path functions require
  access to the operations internals, and some fs operations require access
  to the path parser.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@329028 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/filesystem/class.path/path.member/path.compare.pass.cpp

@@ -26,6 +26,7 @@
 //
 // size_t hash_value(path const&) noexcept;
 
+
 #include "filesystem_include.hpp"
 #include <type_traits>
 #include <vector>
@@ -35,7 +36,7 @@
 #include "test_iterators.h"
 #include "count_new.hpp"
 #include "filesystem_test_helper.hpp"
-
+#include "verbose_assert.h"
 
 struct PathCompareTest {
   const char* LHS;
@@ -57,8 +58,9 @@ const PathCompareTest CompareTestCases[] =
     {"a/b/c", "b/a/c", -1},
     {"a/b", "a/b/c", -1},
     {"a/b/c", "a/b", 1},
-    {"a/b/", "a/b/.", 0},
-    {"a/b//////", "a/b/////.", 0},
+    {"a/b/", "a/b/.", -1},
+    {"a/b/", "a/b",    1},
+    {"a/b//////", "a/b/////.", -1},
     {"a/.././b", "a///..//.////b", 0},
     {"//foo//bar///baz////", "//foo/bar/baz/", 0}, // duplicate separators
     {"///foo/bar", "/foo/bar", 0}, // "///" is not a root directory
@@ -94,8 +96,13 @@ int main()
       int ret2 = normalize_ret(p1.compare(R));
       int ret3 = normalize_ret(p1.compare(TC.RHS));
       int ret4 = normalize_ret(p1.compare(RV));
-      assert(ret1 == ret2 && ret1 == ret3 && ret1 == ret4);
-      assert(ret1 == E);
+
+      g.release();
+      ASSERT_EQ(ret1, ret2);
+      ASSERT_EQ(ret1, ret3);
+      ASSERT_EQ(ret1, ret4);
+      ASSERT_EQ(ret1, E)
+          << DISPLAY(TC.LHS) << DISPLAY(TC.RHS);
 
       // check signatures
       ASSERT_NOEXCEPT(p1.compare(p2));