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

benchmarks/GenerateInput.hpp

@@ -29,14 +29,16 @@ inline std::default_random_engine& getRandomEngine() {
     return RandEngine;
 }
 
+
 inline char getRandomChar() {
     std::uniform_int_distribution<> LettersDist(0, LettersSize-1);
     return Letters[LettersDist(getRandomEngine())];
 }
 
 template <class IntT>
-inline IntT getRandomInteger() {
-    std::uniform_int_distribution<IntT> dist;
+inline IntT getRandomInteger(IntT Min = 0,
+                             IntT Max = std::numeric_limits<IntT>::max()) {
+    std::uniform_int_distribution<IntT> dist(Min, Max);
     return dist(getRandomEngine());
 }