Rollup merge of #69379 - jumbatm:llvm-sigsegv, r=pnkfelix

Rollup merge of #69379 - jumbatm:llvm-sigsegv, r=pnkfelix

Fail on multiple declarations of `main`.

Closes #67946.

Previously, when inserting the entry function, we only checked for
duplicate _definitions_ of `main`.  However, it's possible to cause
problems even only having a duplicate _declaration_. For example,
shadowing `main` using an extern block isn't caught by the current
check, and causes an assertion failure down the line in in LLVM code.

r? @pnkfelix