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

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243069 91177308-0d34-0410-b5e6-96231b3b80d8