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.

llvm-svn: 291403