[Sema][NFC] SequenceChecker: Add tests for references/members, and prepare for the...

[Sema][NFC] SequenceChecker: Add tests for references/members, and prepare for the C++17 tests

Add some tests for unsequenced operations with members and references.
For now most of it is unhandled but it shows what work needs to be done.

Also merge the tests for the C++17 sequencing rules in warn-unsequenced.cpp
since we want to make sure that the appropriate warnings are still present
in C++17 without duplicating the whole content of warn-unsequenced.cpp.

llvm-svn: 354151