[clang] [Clang] strengthen checks for 'main' function to meet [basic.start.main] p2 requirements (PR #101853)

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@@ -990,6 +990,8 @@ def warn_main_redefined : Warning<"variable named 'main' with external linkage "
     "has undefined behavior">, InGroup<Main>;
 def ext_main_used : Extension<
     "referring to 'main' within an expression is a Clang extension">, InGroup<Main>;
+def err_main_invalid_linkage_specification : ExtWarn<
+  "'main' cannot have linkage specification 'extern \"C\"'">, InGroup<Main>;
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Sirraide wrote:

```suggestion
  "'main' should not be extern \"C\"">, InGroup<Main>;
```
I think something like this would be better imo; I don’t like ‘cannot’ since we still accept the code, so it’s not technically correct that it *cannot* have a linkage specification (it can’t according to the standard, but only EDG currently diagnoses this, and even then it’s just a warning), it just really *shouldn’t*.

https://github.com/llvm/llvm-project/pull/101853