Fix the previous (short lived:-) FIXME.

I didn't realize that GCC considers this a hard error (I thought it was built-in).

Since it's not, we should simply emit an error.

[dylan:~/llvm/tools/clang] admin% cc -c trivial.m
trivial.m:6: error: cannot find interface declaration for 'NSConstantString'

[administrators-powerbook59:~/llvm/tools/clang] admin% ../../Debug/bin/clang trivial.m 
trivial.m:6:16: error: cannot find interface declaration for 'NSConstantString'
NSString *s = @"123";
               ^
1 diagnostic generated.

llvm-svn: 43157