[flang] Refine tokenization trick that hid macro name (#121990)

In order to properly expose the Hollerith editing item in something like
FORMAT(3I9HHOLLERITH) as its own token, the tokenization routine in the
prescanner has special handling for digit strings followed by letters
("3I" above). This handler's effects are too broad, and prevent a macro
name from being recognized as such in a reported bug; make the test for
a hidden Hollerith more precise.

Fixes llvm/llvm-project#121931.

Author: klausler

flang/lib/Parser/prescan.cpp

@@ -709,9 +709,22 @@ bool Prescanner::NextToken(TokenSequence &tokens) {
       QuotedCharacterLiteral(tokens, start);
     } else if (IsLetter(*at_) && !preventHollerith_ &&
         parenthesisNesting_ > 0) {
-      // Handles FORMAT(3I9HHOLLERITH) by skipping over the first I so that
-      // we don't misrecognize I9HOLLERITH as an identifier in the next case.
-      EmitCharAndAdvance(tokens, *at_);
+      const char *p{at_};
+      int digits{0};
+      for (;; ++digits) {
+        ++p;
+        if (InFixedFormSource()) {
+          p = SkipWhiteSpace(p);
+        }
+        if (!IsDecimalDigit(*p)) {
+          break;
+        }
+      }
+      if (digits > 0 && (*p == 'h' || *p == 'H')) {
+        // Handles FORMAT(3I9HHOLLERITH) by skipping over the first I so that
+        // we don't misrecognize I9HOLLERITH as an identifier in the next case.
+        EmitCharAndAdvance(tokens, *at_);
+      }
     }
     preventHollerith_ = false;
   } else if (*at_ == '.') {

flang/test/Preprocessing/bug129131.F

@@ -0,0 +1,5 @@
+! RUN: %flang -fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+! CHECK: PRINT *, 2_4
+#define a ,3
+      print *, mod(5 a)
+      end