sema: do checked cast when resolving aggregate size

[dweiller]

Fixes #17664, fixes #23511.

When setting the layout for a struct or union, we cannot do an unchecked @intCast as it is possible the sum of field sizes is greater than std.math.maxInt(u32). This PR adds a new compile error when the resolved size of a union or struct would be greater than std.math.maxInt(u32) to tell the user that their union or struct is too large.

I haven't looked into the reason why, but the compiler seems fine to have tuple structs with sizes up to std.math.maxInt(usize). This feels somewhat inconsistent on 64 bit machines, but this PR is not aiming to change anything about what sizes for aggregate types are supported.

Feel free to bikeshed the wording of the new compile error, it currently looks like:

test/cases/compile_errors/aggregate_too_large.zig:5:11: error: struct layout requires unsupported size: maximum is 4294967295, got 4294967296
const T = struct {
          ^~~~~~