[Sema] Fix crash-on-invalid-code issue:

@class I;
@implementation I(cat) // crashes here
@end

rdar://10968158

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151931 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed