cmCommandArgumentParserHelper: rework input handling

Old implementation uses involved Flex input management technique that
requires usage of obsolete YY_INPUT macro. This causes a lot of useless
allocations and byte-by-byte scanning. New implementation avoids those
hacks, it uses yy_scan_string() API to setup Flex input. Also it fixes
reporting of syntax error position and corresponding tests.

Source/cmCommandArgumentParserHelper.h

@@ -25,7 +25,7 @@ public:
   cmCommandArgumentParserHelper& operator=(
     cmCommandArgumentParserHelper const&) = delete;
 
-  int ParseString(const char* str, int verb);
+  int ParseString(std::string const& str, int verb);
 
   // For the lexer:
   void AllocateParserType(cmCommandArgumentParserHelper::ParserType* pt,
@@ -33,7 +33,6 @@ public:
   bool HandleEscapeSymbol(cmCommandArgumentParserHelper::ParserType* pt,
                           char symbol);
 
-  int LexInput(char* buf, int maxlen);
   void Error(const char* str);
 
   // For yacc
@@ -46,6 +45,8 @@ public:
 
   void SetMakefile(const cmMakefile* mf);
 
+  void UpdateInputPosition(int tokenLength);
+
   std::string& GetResult() { return this->Result; }
 
   void SetLineFile(long line, const char* file);
@@ -57,8 +58,9 @@ public:
   const char* GetError() { return this->ErrorString.c_str(); }
 
 private:
-  std::string::size_type InputBufferPos;
-  std::string InputBuffer;
+  std::string::size_type InputBufferPos{ 1 };
+  std::string::size_type LastTokenLength{};
+  std::string::size_type InputSize{};
   std::vector<char> OutputBuffer;
 
   void Print(const char* place, const char* str);
@@ -75,7 +77,6 @@ private:
   std::string ErrorString;
   const char* FileName;
   long FileLine;
-  int CurrentLine;
   int Verbose;
   bool EscapeQuotes;
   bool NoEscapeMode;