Sema: resolve lazy align in reifyStruct

Closes #12786

src/Sema.zig

@@ -17406,7 +17406,7 @@ fn reifyStruct(
         if (!try sema.intFitsInType(block, src, alignment_val, Type.u32, null)) {
             return sema.fail(block, src, "alignment must fit in 'u32'", .{});
         }
-        const abi_align = @intCast(u29, alignment_val.toUnsignedInt(target));
+        const abi_align = @intCast(u29, (try alignment_val.getUnsignedIntAdvanced(target, sema.kit(block, src))).?);
 
         const field_name = try name_val.toAllocatedBytes(
             Type.initTag(.const_slice_u8),

test/behavior.zig

@@ -87,6 +87,7 @@ test {
     _ = @import("behavior/bugs/12486.zig");
     _ = @import("behavior/bugs/12680.zig");
     _ = @import("behavior/bugs/12776.zig");
+    _ = @import("behavior/bugs/12786.zig");
     _ = @import("behavior/bugs/12794.zig");
     _ = @import("behavior/byteswap.zig");
     _ = @import("behavior/byval_arg_var.zig");

test/behavior/bugs/12786.zig

@@ -0,0 +1,28 @@
+const std = @import("std");
+
+fn NamespacedGlobals(comptime modules: anytype) type {
+    return @Type(.{
+        .Struct = .{
+            .layout = .Auto,
+            .is_tuple = false,
+            .fields = &.{
+                .{
+                    .name = "globals",
+                    .field_type = modules.mach.globals,
+                    .default_value = null,
+                    .is_comptime = false,
+                    .alignment = @alignOf(modules.mach.globals),
+                },
+            },
+            .decls = &[_]std.builtin.Type.Declaration{},
+        },
+    });
+}
+
+test {
+    _ = NamespacedGlobals(.{
+        .mach = .{
+            .globals = struct {},
+        },
+    });
+}