[libcxx] [test] Remove workaround for C1XX conversion-to-nullptr bug

[CaseyCarter]

Summary

VSO#391542 "Types can't be convertible to nullptr_t," which will be fixed in the next release of Visual C++.

Also put internal bug numbers on the workarounds in test_workarounds.h for ease of correlation.

[STL_MSFT]

test/support/test_workarounds.h

21

Do you need to go update the tests that were using this?

[CaseyCarter]

Commit the change to the affected test code as well.

[CaseyCarter]

test/support/test_workarounds.h

21

Yes, thanks, poisoned_hash_helper.hpp didn't make it into the commit.

[EricWF]

LGTM.

[EricWF]

@CaseyCarter Changes like this are good for post-commit review (ie just commit it)