[clang] [SSAF] Extract the virtual method override relation per TU (PR #213316)

[Ziqing Luo via cfe-commits]

https://github.com/ziqingluo-90 commented:

I'd like to see a test where class A inherits class B and B inherits class C.   The base class C has a virtual function `f` being overridden by both A and B.  Is `C::f` in `A::f`'s overridden methods?

On a separate note: each of our SSAF unit test files has its own ad-hoc utility functions for conveniently getting an entityId or an AST node from a readable name. At some point, we may want to have a good general solution for this. Maybe all of them should use ParsedAST.h.

https://github.com/llvm/llvm-project/pull/213316