[clang] [C++26] [Contracts] Initial parser support (PR #221139)

[Chuanqi Xu via cfe-commits]

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+// RUN: %clang_cc1 -std=c17 -fcontracts -fsyntax-only -verify %s
+
+// Contracts are a C++ feature. Enabling the parser option in C mode must not
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ChuanqiXu9 wrote:

`pre` and `post` are contextual keywords. I've added a test to use them in C++. `constract_assert` was contextual keywords. But @cor3ntin prefer to make it unconditional.

https://github.com/llvm/llvm-project/pull/221139