[clang] [clang] Add test for CWG2811 "Clarify "use" of main" (PR #96168)

[Aaron Ballman via cfe-commits]

================
@@ -964,7 +964,7 @@ def err_main_global_variable :
 def warn_main_redefined : Warning<"variable named 'main' with external linkage "
     "has undefined behavior">, InGroup<Main>;
 def ext_main_used : Extension<
-  "ISO C++ does not allow 'main' to be used by a program">, InGroup<Main>;
+  "expressions that refer to 'main' are an extension">, InGroup<Main>;
----------------
AaronBallman wrote:

```suggestion
  "expressions that refer to 'main' are a Clang extension">, InGroup<Main>;
```
I think there's work to be done for full conformance here though. For example, this DR clarified that the following code is valid, but we still diagnose it as an extension:
```
int main() {}
decltype(main) maine;
```
https://godbolt.org/z/nn8rd483Y

https://github.com/llvm/llvm-project/pull/96168