[llvm] [TableGen] Use SmallVectors for preprocessor include stack. NFC. (PR #121571)

[Min-Yih Hsu via llvm-commits]

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@@ -1055,20 +1048,16 @@ bool TGLexer::prepSkipDirectiveEnd() {
 }
 
 bool TGLexer::prepIsProcessingEnabled() {
-  for (const PreprocessorControlDesc &I :
-       llvm::reverse(*PrepIncludeStack.back()))
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mshockwave wrote:

I have no idea why it was using `llvm::reverse` here. Maybe the last element usually has a higher chance to bail out?

https://github.com/llvm/llvm-project/pull/121571