[libc++] Qualifies ptrdiff_t and max_align_t.

This has been done using the following commands
  find libcxx/test -type f -exec perl -pi -e 's|^([^/]+?)((?<!::)ptrdiff_t)|\1std::\2|' \{} \;
  find libcxx/test -type f -exec perl -pi -e 's|^([^/]+?)((?<!::)max_align_t)|\1std::\2|' \{} \;

The std module doesn't export declarations in the global namespaace.,
This is a preparation for that module.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D146550