[ODRHash] Stop hashing `ObjCMethodDecl::isOverriding` as it doesn't capture...

[ODRHash] Stop hashing `ObjCMethodDecl::isOverriding` as it doesn't capture inherent method quality.

`isOverriding` depends on the surrounding code and not on the method
itself. That's why it can be different in different modules. And
mismatches shouldn't be an error.

rdar://109481753

Differential Revision: https://reviews.llvm.org/D154459