Add extension to always default-initialize nullptr_t.

Core issue 1013 suggests that having an uninitialied std::nullptr_t be
UB is a bit foolish, since there is only a single valid value. This DR
reports that DR616 fixes it, which does so by making lvalue-to-rvalue
conversions from nullptr_t be equal to nullptr.

However, just implementing that results in warnings/etc in many places.
In order to fix all situations where nullptr_t would seem uninitialized,
this patch instead (as an otherwise transparent extension) default
initializes uninitialized VarDecls of nullptr_t.

Differential Revision: https://reviews.llvm.org/D53713

Change-Id: I84d72a9290054fa55341e8cbdac43c8e7f25b885
llvm-svn: 349201