[llvm] [TableGen] Check for EOF in prepSkipRegion to avoid preprocessor crash. (PR #208961)

[David Spickett via llvm-commits]

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DavidSpickett wrote:

Can you use FileCheck to check what the error message is? Even if it's incorrect at the moment, we can check for what it currently emits.

https://github.com/llvm/llvm-project/pull/208961