ThreadSafetyAnalysis: Don't crash when trying to analyze objc methods.

The thread safety analysis isn't very useful in ObjC (you can't annotate
ObjC classes or methods) but we can still analyze the actual code and
show violations in usage of C/C++ functions.

Fixes PR19541, which does not use thread safety attributes but crashes
with -Weverything.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208436 91177308-0d34-0410-b5e6-96231b3b80d8