[TableGen] Simplify prepSkipToLineEnd for preprocessing

The MemoryBuffer is created using `RequiresNullTerminator`, so we can
safely skip the `CurPtr != CurBuf.end()` check. The redundant check
causes a cppcheck report. In addition, elsewhere, including `*CurPtr ==
'#'` below, makes the null terminator assumption as well.

Close llvm#81120

Author: MaskRay

llvm/lib/TableGen/TGLexer.cpp

@@ -849,7 +849,8 @@ bool TGLexer::prepSkipRegion(bool MustNeverBeFalse) {
 
   do {
     // Skip all symbols to the line end.
-    prepSkipToLineEnd();
+    while (*CurPtr != '\n')
+      ++CurPtr;
 
     // Find the first non-whitespace symbol in the next line(s).
     if (!prepSkipLineBegin())
@@ -1032,11 +1033,6 @@ bool TGLexer::prepSkipDirectiveEnd() {
   return true;
 }
 
-void TGLexer::prepSkipToLineEnd() {
-  while (*CurPtr != '\n' && *CurPtr != '\r' && CurPtr != CurBuf.end())
-    ++CurPtr;
-}
-
 bool TGLexer::prepIsProcessingEnabled() {
   for (const PreprocessorControlDesc &I :
        llvm::reverse(*PrepIncludeStack.back()))

llvm/lib/TableGen/TGLexer.h

@@ -467,11 +467,6 @@ class TGLexer {
   // directive.
   bool prepSkipDirectiveEnd();
 
-  // Skip all symbols to the end of the line/file.
-  // The method adjusts CurPtr, so that it points to either new line
-  // symbol in the current line or the buffer end.
-  void prepSkipToLineEnd();
-
   // Return true, if the current preprocessor control stack is such that
   // we should allow lexer to process the next token, false - otherwise.
   //