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

[Mital Ashok via cfe-commits]

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@@ -990,6 +990,9 @@ 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_invalid_linkage_specification : Extension<
+  "invalid linkage specification "
+  "'extern \"%select{C|C++}0\"'">;
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MitalAshok wrote:

This doesn't say why it's invalid. `'main' cannot have linkage specification 'extern "C(++)"'`?

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