[Flang] Nested directives are comments (#166348)

Directives cannot be nested. A directive sentinel that appears within
another directive should be ignored, and instead fall back to be treated
as a line comment.

Fixes: #165874

flang/lib/Parser/prescan.cpp

@@ -557,7 +557,7 @@ bool Prescanner::MustSkipToEndOfLine() const {
     return true; // skip over ignored columns in right margin (73:80)
   } else if (*at_ == '!' && !inCharLiteral_ &&
       (!inFixedForm_ || tabInCurrentLine_ || column_ != 6)) {
-    return !IsCompilerDirectiveSentinel(at_ + 1);
+    return InCompilerDirective() || !IsCompilerDirectiveSentinel(at_ + 1);
   } else {
     return false;
   }

flang/test/Parser/OpenMP/nested-directive.f90

@@ -0,0 +1,7 @@
+! RUN: %flang_fc1 -fdebug-unparse -fopenmp %s 2>&1 | FileCheck %s --match-full-lines
+
+subroutine func
+  implicit none
+! CHECK: !$OMP NOTHING
+  !$omp nothing !$omp Cannot nest directives inside directives; must be interpreted as a comment
+end subroutine func