stage2: coerce [*:0]u8 to other valid pointer types

This makes the following work properly (as it does in stage1, too):

  var zero_ptr: [*:0]const u8 = undefined;
  var no_zero_ptr: [*]const u8 = zero_ptr;

Prior to this this would fail with an "expected type" error since
coercion failed.

src/Sema.zig

@@ -19939,6 +19939,7 @@ fn typePtrOrOptionalPtrTy(
         .many_mut_pointer,
         .manyptr_u8,
         .manyptr_const_u8,
+        .manyptr_const_u8_sentinel_0,
         => return ty,
 
         .pointer => switch (ty.ptrSize()) {