Fix 3 bugs in filesystem tests and implementation.

This patch fixes the following bugs, all of which were discovered while
testing a 32 bit build on a 64 bit machine.

* path.itr/iterator.pass.cpp has undefined behavior.
  'path::iterator' stashes the value of the element inside the iterator.
  This violates the BiDirIterator requirements but is allowed for path::iterator.
  However this means that using reverse_iterator<path::iterator> has undefined
  behavior because it assumes that 'Iter tmp = it; return *tmp' will not create
  a dangling reference. However it does, and this caused this particular test
  to fail.

* path.native.obs/string_alloc.pass.cpp tested the SSO with a long string.
  On 32 bit builds std::wstring only has the SSO for strings of size 2. The
  test was using a string of size 4.

* fs.op.space/space.pass.cpp had overflows while calculating the expected values.
  The fix here is to convert the statvfs data members to std::uintmax_t before
  multiplying them. The internal implementation already does this but the tests
  needed to do it as well.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@273078 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/filesystem_test_helper.hpp

@@ -361,6 +361,22 @@ bool checkCollectionsEqual(
     return (start1 == end1 && start2 == end2);
 }
 
+
+template <class Iter1, class Iter2>
+bool checkCollectionsEqualBackwards(
+    Iter1 const start1, Iter1 end1
+  , Iter2 const start2, Iter2 end2
+  )
+{
+    while (start1 != end1 && start2 != end2) {
+        --end1; --end2;
+        if (*end1 != *end2) {
+            return false;
+        }
+    }
+    return (start1 == end1 && start2 == end2);
+}
+
 // We often need to test that the error_code was cleared if no error occurs
 // this function returns a error_code which is set to an error that will
 // never be returned by the filesystem functions.