compiler: audit uses of ptr.addr in the frontend

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

Author: mlugg