Currently we're initializing the vtable pointers of a class only after

Currently we're initializing the vtable pointers of a class only after
the bases are completely initialized.  This won't work --- base
initializer expressions can rely on the vtables having been set up.
Check for uses of 'this' in the initializers and force a vtable
initialization if found.

This might not be good enough;  we might need to extend this to handle
the possibility of arbitrary code finding an external reference to this
(not yet completely-constructed!) object and accessing through it,
in which case we'll probably find ourselves doing a lot more unnecessary
stores.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@114153 91177308-0d34-0410-b5e6-96231b3b80d8