[Flang] preprocessor bug with OpenMP sentinels in fixed-format source

When fixed-format .F source code with OpenMP directives exists, but OpenMP isn't used, Flang isn't preprocessing the source code correctly. This impacts CMake + Ninja, with the root cause seeming to be a bug in Flang revealed by that common use case.
The bottom of this issue has minimal reproducer code. This occurs across all versions of LLVM Flang I've tried over a couple years.

```sh
src = "flang_mwe.F"
pp = "flang_mwe-pp.f"

flang -c $src # OK
flang -cpp -E $src -o $pp # emits warning
flang -c $pp # errors
```

### Expected

1. direct compile succeeds
2. preprocess succeeds
3. compile succeds

The expected source preprocessed snippet is the following. This is like what Gfortran correctly generates.

```fortran
call mysub(a, b
!$ & ,dummy
& )
```

### Actual

1. Direct compile succeeds.
2. Preprocess gives warning and incorrect syntax generated
3. Compile of preprocessed file fails with unmatched parentheses.

The Flang preprocess step generates this syntax with a warning, which is incorrect generated syntax.

```fortran
call mysub(a, b
!$ &,dummy&
)
```

## file "flang_mwe.F"

```fortran
program mwe

call mysub(a, b
!$ & ,dummy
& )

contains

subroutine mysub(a, b, dummy)
real :: a, b
real, optional :: dummy
end subroutine mysub

end program
```

Start by running the commands with flang_mwe.F and compare Flang's preprocessed output with the expected snippet and the GFortran-like form shown in the issue. Trace fixed-format preprocessing of the OpenMP sentinel and continuation lines; done means preprocessing emits valid syntax and the resulting file compiles successfully.

Written by the indexing model from the issue text.