Sema: add error for capturing a runtime value outside of function scope

Closes #13104

test/cases/compile_errors/accessing_runtime_paramter_outside_function_scope.zig

@@ -0,0 +1,12 @@
+export fn entry(y: u8) void {
+    const Thing = struct {
+        y: u8 = y,
+    };
+    _ = @sizeOf(Thing);
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:17: error: 'y' not accessible outside function scope