[clang] Add test cases for multiversion function overload scenarios in C.

This change adds test cases to validate diagnostics for overloaded sets
that contain declarations of multiversion functions. Many of the added test
cases exercise declarations that are intended to be valid. Others are
intended to be valid if and when restrictions on multiversion functions
being declared with the overloadable attribute are lifted.

Several of the new test cases currently trigger the following assertion
failure in SemaDecl.cpp; the relevant test is therefore marked as an
expected failure pending a fix.
```
Assertion `(Previous.empty() || llvm::any_of(Previous, [](const NamedDecl *ND) { return ND->hasAttr(); })) && "Non-redecls shouldn't happen without overloadable present"' failed.
```

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D121954