[clang] [Clang] Avoid assertion failure for initialized extern aliases (PR #223124)

[Aaron Ballman via cfe-commits]

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+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fsyntax-only -verify %s
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AaronBallman wrote:

Rather than introduce a new file for this, how painful is it to add an other `RUN` line to `clang/test/Sema/alias-redefinition.c` that does `-x c++` to additionally run in C++ mode? Then you can put the C++-specific code into a `#ifdef __cplusplus` block so it doesn't trigger unrelated diagnostics for the C RUN line.

(If it requires updating many diagnostics in the file, then it's not worth the effort. But if possible, it'd be nice to keep the redefinition tests in the same file.)

https://github.com/llvm/llvm-project/pull/223124