Teach the type checker about "id". This removes the following bogus warning...

[dylan:~/llvm/tools/clang] admin% ../../Debug/bin/clang t.m
t.m:29:18: warning: incompatible pointer types assigning 'id' to 'NSString *'
    resultString = [[NSString alloc] initWithFormat:0 arguments:0];
    ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

llvm-svn: 42975