Implement code completion for Objective-C class message sends that are

missing the opening bracket '[', e.g.,

  NSArray <CC>

at function scope. Previously, we would only give trivial completions
(const, volatile, etc.), because we're in a "declaration name"
scope. Now, we also provide completions for class methods of NSArray,
e.g.,

  alloc

Note that we already had support for this after the first argument,
e.g.,

  NSArray method:x <CC>

would get code completion for class methods of NSArray whose selector
starts with "method:". This was already present because we recover
as if NSArray method:x were a class message send missing the opening
bracket (which was committed in r114057).

llvm-svn: 114078