[CodeGen] Use the non-virtual alignment when emitting the base

constructor.

Previously, clang would emit an over-aligned (16-byte) store to
initialize B::x in B's base constructor when compiling the following
code:

struct A {
  __attribute__((aligned(16))) double data1;
};

struct B : public virtual A {
  B() : x(123) {}
  double a;
  int x;
};

struct C : public virtual B {};

void test() { B b; C c; }

This was happening because the code in IRGen that does member
initialization was using the alignment of a complete object instead of
the non-virtual alignment.

This commit fixes the bug.

rdar://problem/36382481

Differential Revision: https://reviews.llvm.org/D42521

llvm-svn: 323578