Implement explicit instantiations of member classes of class templates, e.g.,

  template<typename T>
  struct X {
    struct Inner;
  };

  template struct X<int>::Inner;

This change is larger than it looks because it also fixes some
a problem with nested-name-specifiers and tags. We weren't requiring
the DeclContext associated with the scope specifier of a tag to be
complete. Therefore, when looking for something like "struct
X<int>::Inner", we weren't instantiating X<int>. 

This, naturally, uncovered a problem with member pointers, where we
were requiring the left-hand side of a member pointer access
expression (e.g., x->*) to be a complete type. However, this is wrong:
the semantics of this expression does not require a complete type (EDG
agrees).

Stuart vouched for me. Blame him.

llvm-svn: 71756
diff --git a/clang/test/SemaTemplate/instantiate-complete.cpp b/clang/test/SemaTemplate/instantiate-complete.cpp
index 7b43735..babc552 100644
--- a/clang/test/SemaTemplate/instantiate-complete.cpp
+++ b/clang/test/SemaTemplate/instantiate-complete.cpp
@@ -11,8 +11,7 @@
        // expected-error{{data member instantiated with function type 'int (int)'}} \
        // expected-error{{data member instantiated with function type 'char (char)'}} \
        // expected-error{{data member instantiated with function type 'short (short)'}} \
-       // expected-error{{data member instantiated with function type 'float (float)'}} \
-       // expected-error{{data member instantiated with function type 'long (long)'}}
+       // expected-error{{data member instantiated with function type 'float (float)'}}
 };
 
 X<int> f() { return 0; }
@@ -41,7 +40,8 @@
 }
 
 void test_memptr(X<long> *p1, long X<long>::*pm1,
-                 X<long(long)> *p2, long (X<long(long)>::*pm2)(long)) {
+                 X<long(long)> *p2, 
+                 long (X<long(long)>::*pm2)(long)) {
   (void)(p1->*pm1);
-  (void)(p2->*pm2); // expected-note{{in instantiation of template class 'struct X<long (long)>' requested here}}
+  (void)((p2->*pm2)(0));
 }