[TableGen] Fix incorrect handling of nested `#ifndef` directives

[myhsu]

Summary

TableGen's lexer was unable to handle nested #ifndef when the outer #ifdef / #ifndef scope is subject to skip. This was caused by returning the canonicalized token when it should have returned the original one.

Fix #65100 .

llvm/lib/TableGen/TGLexer.cpp

tgtok::TokKind TGLexer::lexPreprocessor(
   if (Kind == tgtok::Ifdef || Kind == tgtok::Ifndef) {
     StringRef MacroName = prepLexMacroName();
     StringRef IfTokName = Kind == tgtok::Ifdef ? "#ifdef" : "#ifndef";
     if (MacroName.empty())
       return ReturnError(TokStart, "Expected macro name after " + IfTokName);
 
     bool MacroIsDefined = DefinedMacros.count(MacroName) != 0;
 
-    // Canonicalize ifndef to ifdef equivalent
-    if (Kind == tgtok::Ifndef) {
+    // Canonicalize ifndef's MacroIsDefined to its ifdef equivalent.
+    if (Kind == tgtok::Ifndef)
       MacroIsDefined = !MacroIsDefined;
-      Kind = tgtok::Ifdef;
-    }
 
     // Regardless of whether we are processing tokens or not,
     // we put the #ifdef control on stack.
+    // Note that MacroIsDefined has been canonicalized against ifdef.
     PrepIncludeStack.back()->push_back(
-        {Kind, MacroIsDefined, SMLoc::getFromPointer(TokStart)});
+        {tgtok::Ifdef, MacroIsDefined, SMLoc::getFromPointer(TokStart)});
 
     if (!prepSkipDirectiveEnd())
       return ReturnError(CurPtr, "Only comments are supported after " +
                                      IfTokName + " NAME");
 
     // If we were not processing tokens before this #ifdef,
     // then just return back to the lines skipping code.
     if (!ReturnNextLiveToken)

llvm/test/TableGen/nested_ifdef.inc

+#ifndef NESTED_IFDEF
+#define NESTED_IFDEF
+
+def foo;
+
+#ifndef HAHA
+def haha;
+#endif
+
+#endif

llvm/test/TableGen/nested_ifdef2.inc

+#ifndef NESTED_IFDEF2
+#define NESTED_IFDEF2
+
+include "nested_ifdef.inc"
+
+def bar;
+
+#endif

llvm/test/TableGen/nested_ifdef_main.td

+// RUN: llvm-tblgen -I %p %s | FileCheck %s
+
+include "nested_ifdef.inc"
+include "nested_ifdef2.inc"
+
+// CHECK: def bar
+// CHECK: def foo
+// CHECK: def haha
+// CHECK: def zoo
+
+def zoo;