Sema: fix empty slice pointer value

We just checked that inst_child_ty was effectively a zero-bit type, so
it is certainly not the non-zero alignment we are looking for.

Closes #15085

src/codegen/c.zig

@@ -1069,7 +1069,7 @@ pub const DeclGen = struct {
                     const extern_fn = val.castTag(.extern_fn).?.data;
                     try dg.renderDeclName(writer, extern_fn.owner_decl, 0);
                 },
-                .int_u64, .one => {
+                .int_u64, .one, .int_big_positive, .lazy_align, .lazy_size => {
                     try writer.writeAll("((");
                     try dg.renderType(writer, ty);
                     return writer.print("){x})", .{try dg.fmtIntLiteral(Type.usize, val, .Other)});

src/codegen/llvm.zig

@@ -3397,7 +3397,7 @@ pub const DeclGen = struct {
                     };
                     return dg.context.constStruct(&fields, fields.len, .False);
                 },
-                .int_u64, .one, .int_big_positive => {
+                .int_u64, .one, .int_big_positive, .lazy_align, .lazy_size => {
                     const llvm_usize = try dg.lowerType(Type.usize);
                     const llvm_int = llvm_usize.constInt(tv.val.toUnsignedInt(target), .False);
                     return llvm_int.constIntToPtr(try dg.lowerType(tv.ty));