Implement parsing for message sends in Objective-C++. Message sends in

Objective-C++ have a more complex grammar than in Objective-C
(surprise!), because

  (1) The receiver of an instance message can be a qualified name such
  as ::I or identity<I>::type.
  (2) Expressions in C++ can start with a type.

The receiver grammar isn't actually ambiguous; it just takes a bit of
work to parse past the type before deciding whether we have a type or
expression. We do this in two places within the grammar: once for
message sends and once when we're determining whether a []'d clause in
an initializer list is a message send or a C99 designated initializer.

This implementation of Objective-C++ message sends contains one known
extension beyond GCC's implementation, which is to permit a
typename-specifier as the receiver type for a class message, e.g.,

  [typename compute_receiver_type<T>::type method];

Note that the same effect can be achieved in GCC by way of a typedef,
e.g.,

  typedef typename computed_receiver_type<T>::type Computed;
  [Computed method];

so this is merely a convenience.

Note also that message sends still cannot involve dependent types or
values.

llvm-svn: 102031