When calling IsCompilerDirectiveSentinel,the prefix comment character need to be skipped.

Fixes #117693

Author: shivaramaarao

flang/lib/Parser/prescan.cpp

@@ -511,7 +511,7 @@ bool Prescanner::MustSkipToEndOfLine() const {
   if (inFixedForm_ && column_ > fixedFormColumnLimit_ && !tabInCurrentLine_) {
     return true; // skip over ignored columns in right margin (73:80)
   } else if (*at_ == '!' && !inCharLiteral_) {
-    return !IsCompilerDirectiveSentinel(at_);
+    return !IsCompilerDirectiveSentinel(at_+1);
   } else {
     return false;
   }

flang/test/Preprocessing/bug117693.f90

@@ -0,0 +1,14 @@
+! RUN: %flang -fopenmp -E %s 2>&1 | FileCheck %s
+!CHECK: !$OMP DO SCHEDULE(STATIC)
+program main
+IMPLICIT NONE
+INTEGER:: I
+#define OMPSUPPORT
+!$ INTEGER :: omp_id
+!$OMP PARALLEL DO
+OMPSUPPORT !$OMP DO SCHEDULE(STATIC)
+DO I=1,100
+print *, omp_id
+ENDDO
+!$OMP END PARALLEL DO
+end program