bit_field: diagnose a missing field name in type_field_bit_offset / type_field_bit_size

[kalsprite]

Both builtins search type->BitField.fields with accumulators init to 0 and no not-found branch, so a name that does not exist yields 0.

BF :: bit_field u32 {a: u8 | 3, b: u8 | 5}
#assert(intrinsics.type_field_bit_offset(BF, "nope") == 0)  // accepted
#assert(intrinsics.type_field_bit_offset(BF, "a")    == 0)  // ...and so is this