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

[via cfe-commits]

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

Also add a test for this (at the top level w/o any linkage-spec-decl) if we don’t already have one:
```
int main;
```
which is also invalid; probably in a separate file so it doesn’t complain about redeclarations

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