[flang] Treat conditional comments as comments (#165881)

An OpenMP, OpenACC, or CUDA conditional line should be treated as a
comment when that's what its payload contains, not as a conditional
source line that will confuse the parser when it is indeed just a
comment.

flang/lib/Parser/prescan.cpp

@@ -1642,6 +1642,17 @@ Prescanner::IsFixedFormCompilerDirectiveLine(const char *start) const {
       // This is a Continuation line, not an initial directive line.
       return std::nullopt;
     }
+    ++column, ++p;
+  }
+  if (isOpenMPConditional) {
+    for (; column <= fixedFormColumnLimit_; ++column, ++p) {
+      if (IsSpaceOrTab(p)) {
+      } else if (*p == '!') {
+        return std::nullopt; // !$    ! is a comment, not a directive
+      } else {
+        break;
+      }
+    }
   }
   if (const char *ss{IsCompilerDirectiveSentinel(
           sentinel, static_cast<std::size_t>(sp - sentinel))}) {
@@ -1657,8 +1668,17 @@ Prescanner::IsFreeFormCompilerDirectiveLine(const char *start) const {
       p && *p++ == '!') {
     if (auto maybePair{IsCompilerDirectiveSentinel(p)}) {
       auto offset{static_cast<std::size_t>(p - start - 1)};
-      return {LineClassification{LineClassification::Kind::CompilerDirective,
-          offset, maybePair->first}};
+      const char *sentinel{maybePair->first};
+      if ((sentinel[0] == '$' && sentinel[1] == '\0') || sentinel[1] == '@') {
+        if (const char *comment{IsFreeFormComment(maybePair->second)}) {
+          if (*comment == '!') {
+            // Conditional line comment - treat as comment
+            return std::nullopt;
+          }
+        }
+      }
+      return {LineClassification{
+          LineClassification::Kind::CompilerDirective, offset, sentinel}};
     }
   }
   return std::nullopt;

flang/test/Preprocessing/bug136845.F

@@ -18,7 +18,6 @@ c$   !1                                                                 B
 *$1   continue
       end
 
-!PREPRO:!$   &
 !PREPRO:              continue
 !PREPRO:      k=0
 !PREPRO:      k=0

flang/test/Preprocessing/cond-comment.f

@@ -0,0 +1,5 @@
+!RUN: %flang_fc1 -fopenmp -fdebug-unparse %s 2>&1 | FileCheck %s
+!CHECK: END
+!CHECK-NOT: error:
+      end
+c$      !

flang/test/Preprocessing/cond-comment.f90

@@ -0,0 +1,5 @@
+!RUN: %flang_fc1 -fopenmp -fdebug-unparse %s 2>&1 | FileCheck %s
+!CHECK: END
+!CHECK-NOT: error:
+end
+!$ !