[completion] Add completions for @"..." and @(...), and tidy up @[] and @{}.

Specifically, @[] and @{} didn't have a type associated with them; we now
use "NSArray *" and "NSDictionary *", respectively. @"" has the type
"NSString *". @(), unfortunately, has type "id", since it (currently) may
be either an NSNumber or an NSString.

Add a test for all the Objective-C at-expression completions.

<rdar://problem/11507708&11507668&11507711>

llvm-svn: 158533