CFI: Cover each InstanceKind explicitly and reorganize UI tests

[rcvalle]

• Changes transform_instance to cover each InstanceKind (and ShimKind) explicitly, similarly to how encoding is done, so the intent is expressed clearly and it is known when an instance is handled (or not) and the side effects of it (also clearly), instead of relying on fallthrough behavior. This also makes adding a new InstanceKind (or ShimKind) result in a compile-time error until it is explicitly handled. No functional changes intended.
• Reorganizes the CFI and KCFI UI tests under the cfi and kcfi directories similarly to how the CFI codegen tests are organized: each test now tests a type or a language construct or feature that is handled by the CFI and KCFI transform or encoding, with complete coverage of what can be exercised at run time, and is named works-with-, and tests that covered both CFI and KCFI using revisions are split into separate tests. It also adds tests for the types, language constructs, and features that were not covered (i.e., intrinsics, thread locals, builtin Clone and FnPtr implementations, pattern types, the never type, extern types, C variadics, and the generalize-pointers and normalize-integers options), removes the tests that became redundant, and changes the drop tests to run-pass, as they now pass at run time. Tests for flags/options are unchanged (only moved into the cfi and kcfi directories when at the top level).

r? @bjorn3