Disallow casts to null-terminated pointers in checked scopes.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#391

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We should not allow casts from unchecked, ptr, or array_ptr values to null-terminated in checked scopes. Those pointers might not pointed to a null-terminated array, or the null terminator may be overwritten.

[secure-sw-dev-bot]

Comment from @dtarditi:

I believe we are already disallow implicit casts. This work item is to disallow explicit casts.

[secure-sw-dev-bot]

Comment from @mgrang:

This was fixed in checkedc/checkedc-clang#626 and tests were added in checkedc/checkedc#367.