Add Peek

Add a peek function to let the Lexer look at a character arbitrarily
far ahead in the stream without consuming anything.  We need this to
disambiguate numbers and operands of a paste operation.  For example:

def foo#8i

Without lookahead the lexer will treat '8' as a number rather than as
part of a string to be pasted to form an identifier.

llvm-svn: 142512

lib/TableGen/TGLexer.cpp

@@ -80,6 +80,10 @@ int TGLexer::getNextChar() {
   }  
 }
 
+int TGLexer::peekNextChar(int Index) {
+  return *(CurPtr + Index);
+}
+
 tgtok::TokKind TGLexer::LexToken() {
   TokStart = CurPtr;
   // This always consumes at least one character.

lib/TableGen/TGLexer.h

@@ -109,6 +109,7 @@ private:
   tgtok::TokKind ReturnError(const char *Loc, const Twine &Msg);
   
   int getNextChar();
+  int peekNextChar(int Index);
   void SkipBCPLComment();
   bool SkipCComment();
   tgtok::TokKind LexIdentifier();