Fix a couples of issues in format strings checking.

PR 10274: format function attribute with the NSString archetype yields no compiler warnings
PR 10275: format function attribute isn't checked in Objective-C methods



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148324 91177308-0d34-0410-b5e6-96231b3b80d8