Don't require nullability on template parameters in typedefs.

Don't require nullability on template parameters in typedefs.

Previously the following code would warn on the use of "T":

  template <typename T>
  struct X {
    typedef T *type;
  };

...because nullability is /allowed/ on template parameters (because
they could be pointers). (Actually putting nullability on this use of
'T' will of course break if the argument is a non-pointer type.)

This fix doesn't handle the case where a template parameter is used
/outside/ of a typedef. That seems trickier, especially in parameter
position.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285856 91177308-0d34-0410-b5e6-96231b3b80d8