Fix the equal-vector-size rule for reinterpret_casts in C++

to consider the storage size of the vector instead of its
sizeof.  In other words, ban <3 x int> to <4 x int> casts,
which produced invalid IR anyway.

Also, attempt to be a little more rigorous, or at least
explicit, about when enums are allowed in these casts.

rdar://21901132

llvm-svn: 243069