This commit also performs some refactors to `TypedValue.print` in
preparation for improved comptime pointer access logic. Once that logic
exists, `TypedValue.print` can use Sema to access pointers for more
helpful printing.
This commit also implements proposal #19435, because the existing logic
there relied on some blatantly incorrect code in `Value.sliceLen`.
Resolves: #19435
152a2ceaf7