[clang] [clang][Parser] Warn when the body of expansion statement is not a compound statement (PR #209229)

[Aaron Ballman via cfe-commits]

================
@@ -457,6 +457,9 @@ def err_expansion_stmt_requires_cxx2c : Error<
   "expansion statements are only supported in C++2c">;
 def err_for_template : Error<
   "'for template' is invalid; use 'template for' instead">;
+def ext_expansion_stmt_missing_braces : Extension<
+  "ISO C++ requires a compound statement to be the body of an expansion statement">,
----------------
AaronBallman wrote:

```suggestion
  "ISO C++ requires the body of an expansion statement to be a compound statement">,
```
This just seems to read better to me, WDYT?

https://github.com/llvm/llvm-project/pull/209229