Enhance Clang to start instantiating static data member definitions

within class templates when they are necessary to complete the type of
the member. The canonical example is code like:

  template <typename T> struct S {
    static const int arr[];
    static const int x;
    static int f();
  };

  template <typename T> const int S<T>::arr[] = { 1, 2, 3 };
  template <typename T> const int S<T>::x = sizeof(arr) / sizeof(arr[0]);
  template <typename T> int S<T>::f() { return x; }

  int x = S<int>::f();

We need to instantiate S<T>::arr's definition to pick up its initializer
and complete the array type. This involves new code to specially handle
completing the type of an expression where the type alone is
insufficient. It also requires *updating* the expression with the newly
completed type. Fortunately, all the other infrastructure is already in
Clang to do the instantiation, do the completion, and prune out the
unused bits of code that result from this instantiation.

This addresses the initial bug in PR10001, and will be a step to
fleshing out other cases where we need to work harder to complete an
expression's type. Who knew we still had missing C++03 "features"?

llvm-svn: 132172