[flang] Don't mistakenly tokenize a Hollerith literal from "DO 100 H=..." (bug #58732)

[klausler]

Summary

After tokenizing an identifier, don't allow the next token to be a
Hollerith literal.

Fixes https://github.com/llvm/llvm-project/issues/58732.

flang/lib/Parser/prescan.cpp

if (!inPreprocessorDirective_ && IsDecimalDigit(nch)) {
       while (IsDecimalDigit(EmitCharAndAdvance(tokens, *at_))) {
       }
       ExponentAndKind(tokens);
     } else if (nch == '.' && EmitCharAndAdvance(tokens, '.') == '.') {
       EmitCharAndAdvance(tokens, '.'); // variadic macro definition ellipsis
     }
     preventHollerith_ = false;
   } else if (IsLegalInIdentifier(*at_)) {
-    do {
-    } while (IsLegalInIdentifier(EmitCharAndAdvance(tokens, *at_)));
+    while (IsLegalInIdentifier(EmitCharAndAdvance(tokens, *at_))) {
+    }
+    if (InFixedFormSource()) {
+      SkipSpaces();
+    }
     if ((*at_ == '\'' || *at_ == '"') &&
         tokens.CharAt(tokens.SizeInChars() - 1) == '_') { // kind_"..."
       QuotedCharacterLiteral(tokens, start);
-    }
       preventHollerith_ = false;
+    } else {
+      preventHollerith_ = true; // DO 10 H = ...
+    }
   } else if (*at_ == '*') {
     if (EmitCharAndAdvance(tokens, '*') == '*') {
       EmitCharAndAdvance(tokens, '*');
     } else {
       // Subtle ambiguity:
       //  CHARACTER*2H     declares H because *2 is a kind specifier
       //  DATAC/N*2H  /    is repeated Hollerith
       preventHollerith_ = !slashInCurrentStatement_;

flang/test/Parser/do100h.f

+! RUN: %flang_fc1 -E %s | FileCheck %s
+! Test that Hollerith is not mistakenly tokenized here
+!CHECK: do 100 h=1,10
+      do 100 h=1,10
+100   continue
+      end
+