Apply the changes from FreeBSD r314883:
Pull in r291403 from upstream clang trunk (by Richard Smith):
PR30305: Implement proposed DR resolution to prevent slicing via
inherited constructor.
The rule we use is that a construction of a class type T from an
argument of type U cannot use an inherited constructor if U is the
same as T or is derived from T (or if the initialization would first
convert it to such a type). This (approximately) matches the rule in
use by GCC, and matches the current proposed DR resolution.
Pull in r291955 from upstream clang trunk (by Richard Smith):
(Only the first 15 lines of the commit message are shown above )