[Sema][SVE] Add tests for valid and invalid type usage

This patch adds C and C++ tests for various uses of SVE types.
The tests cover valid uses that are already (correctly) accepted and
invalid uses that are already (correctly) rejected.  Later patches
will expand the tests as they fix other cases.[*]

Some of the tests for invalid uses aren't obviously related to
scalable vectors.  Part of the reason for having them is to make
sure that the quality of the error message doesn't regress once/if
the types are treated as incomplete types.

[*] These later patches all fix invalid uses that are being incorrectly
    accepted.  I don't know of any cases in which valid uses are being
    incorrectly rejected.  In other words, this series is all about
    diagnosing invalid code rather than enabling something new.

Differential Revision: https://reviews.llvm.org/D75571