[clang] Fix typo in comments

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Differential Revision: https://phabricator.intern.facebook.com/D60250786

clang/lib/Lex/Preprocessor.cpp

@@ -988,7 +988,7 @@ void Preprocessor::LexTokensUntilEOF(std::vector<Token> *Tokens) {
 }
 
 /// Lex a header-name token (including one formed from header-name-tokens if
-/// \p AllowConcatenation is \c true).
+/// \p AllowMacroExpansion is \c true).
 ///
 /// \param FilenameTok Filled in with the next token. On success, this will
 ///        be either a header_name token. On failure, it will be whatever other