[libc++] Qualifies intptr_t and uintptr_t.

This has been done using the following command

  find libcxx/test -type f -exec perl -pi -e 's|^([^/]+?)((?<!::)(?<!::u)u?intptr_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/D146643