[flang] Adjust CHECK() so that first token of program can begin with first character.

Original-commit: flang-compiler/f18@2b77e466ff
Reviewed-on: https://github.com/flang-compiler/f18/pull/9
Tree-same-pre-rewrite: false

flang/lib/parser/prescan.cc

@@ -168,7 +168,7 @@ static inline bool IsNameChar(char ch) {
 }
 
 bool Prescanner::NextToken(TokenSequence *tokens) {
-  CHECK(at_ > start_ && at_ < limit_);  // TODO pmk
+  CHECK(at_ >= start_ && at_ < limit_);  // TODO pmk rm?
   if (inFixedForm_) {
     SkipSpaces();
   } else if (*at_ == ' ' || *at_ == '\t') {