[libcxx] [test] Add more tests to tuple_size_structured_bindings.pass.cpp and...

[libcxx] [test] Add more tests to tuple_size_structured_bindings.pass.cpp and make it friendlier to C1XX.

Style/paranoia: 42.1 doesn't have an exact binary representation. Although this doesn't
cause failures, it makes me uncomfortable, so I'm changing it to 42.5.

C1XX rightly warns about unreferenced variables. Adding tests for their values
makes C1XX happy and improves test coverage.

C1XX (somewhat obnoxiously) warns about seeing a struct specialized as a class.
Although the Standard doesn't care, saying struct consistently is better.
(The Standard itself is still inconsistent about whether to depict tuple_element
and tuple_size as structs or classes.)

Fixes D33953.

llvm-svn: 305843