[llvm] [TableGen] Use assert instead of PrintFatalError in TGLexer. NFC. (PR #122303)

[Jay Foad via llvm-commits]

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@@ -1053,10 +1040,6 @@ bool TGLexer::prepIsProcessingEnabled() {
 }
 
 void TGLexer::prepReportPreprocessorStackError() {
-  if (PrepIncludeStack.back().empty())
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jayfoad wrote:

In my opinion the message doesn't add much value. Otherwise we should put `assert(!empty() && "explanation")` before every usage of `.back()`.

https://github.com/llvm/llvm-project/pull/122303