Fix size of flexible array initializers, and re-enable assertions.

In D123649, I got the formula for getFlexibleArrayInitChars slightly
wrong: the flexible array elements can be contained in the tail padding
of the struct.  Fix the formula to account for that.

With the fixed formula, we run into another issue: in some cases, we
were emitting extra padding for flexible arrray initializers. Fix
CGExprConstant so it uses a packed struct when necessary, to avoid this
extra padding.

Differential Revision: https://reviews.llvm.org/D123826