Whenever completing ordinary names for an Objective-C source, also

provide completions for @ keywords. Previously, we only provided
@-completions after an @ was actually typed, which is useful but
probably not the common case.

Also, make sure a few Objective-C 2.0 completions only show up when
Objective-C 2.0 support is enabled (the default).

llvm-svn: 93354