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

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+// RUN: %clang_cc1 -fsyntax-only -verify -pedantic-errors %s
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Sirraide wrote:

I think this was already pointed out, but some tests w/ `extern "C++"` would be nice, as well as tests for when a namespace is contained in a linkage-spec-decl.

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