[clang] [Clang] Introduce __builtin_is_modifiable_lvalue() (PR #132524)

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+// RUN: %clang_cc1 -std=c99 -fsyntax-only -verify %s
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Sirraide wrote:

> Edit: Perhaps it's wanted to make this intrinsic accept VLA (and report `false`) and variably-modified types without evaluating the non-constant array size.

That should definitely be possible. I don’t think we really need to evaluate anything here.

https://github.com/llvm/llvm-project/pull/132524