Add getSentinel helper to Pointer and Array builtin.Types

[xdBronch]

No description provided.

[mlugg]

I think it'd make more sense for the field to be sentinel_ptr and the method sentinel().

[xdBronch]

the breakings will continue until morale improves 🙃

[xdBronch]

hmm im having second thoughts on this asserting that its not null. a decent number of uses of sentinel_ptr are first checking if its null, as does std.meta.sentinel (which i didnt notice existed before :p but i think itd still be favorable to have something builtin like this). thoughts? yeah nevermind i think this is vastly more useful if it returns null

[xdBronch]

so if im not wrong this requires a zig1 update since it changes builtin, correct? assuming this change is deemed worth it, is the correct order of operations something like this?
commit 1. change the field in builtin.zig and sema + any uses of it
commit 2. update zig1.wasm, later to be replaced by a team member
commit 3. add the function and update any places where its useful