This patch contains a number of small portability improvements for the
test suite, making it easier to run the test suite with other standard
library implementations.
- Guard checks for _LIBCPP_HARDENING_MODE to avoid -Wundef
- Avoid defining _LIBCPP_HARDENING_MODE even when no hardening mode is
specified -- we should use the default mode of the library in that case.
- Add missing includes and qualify a few function calls.
- Avoid opening namespace std to forward declare stdlib containers. The
test suite should represent user code, and user code isn't allowed to do
that.
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