[MS ABI] Don't zero-initialize vbptrs in bases

Certain CXXConstructExpr nodes require zero-initialization before a
constructor is called.  We had a bug in the case where the constructor
is called on a virtual base: we zero-initialized the base's vbptr field.
A complementary bug is present in MSVC where no zero-initialization
occurs for the subobject at all.

This fixes PR25370.

llvm-svn: 251783