Fix rejects-valid caused by r261297.

Fix rejects-valid caused by r261297.

r261297 called hasUserProvidedDefaultConstructor() to check if defining a
const object is ok.  This is incorrect for this example:

  struct X { template<typename ...T> X(T...); int n; };
  const X x; // formerly OK, now bogus error

Instead, track if a class has a defaulted default constructor, and disallow
a const object for classes that either have defaulted default constructors or
if they need an implicit constructor.

Bug report and fix approach by Richard Smith, thanks!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261770 91177308-0d34-0410-b5e6-96231b3b80d8