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.

llvm-svn: 273078