[Flang][OpenMP] DECLARE TARGET with INDIRECT clause aborts during lowering

## Description

Flang currently aborts while lowering an OpenMP `DECLARE TARGET` directive that uses the `INDIRECT` clause.

The failing directive is:

```fortran
!$omp declare target enter(func1) indirect(.true.)
```

This is used with a procedure pointer call inside an OpenMP target region:

```fortran
procedure (func), pointer :: ptr1 => func1
character(1) :: val1

!$omp target map(from: val1)
val1 = ptr1()
!$omp end target
```

Here, `ptr1()` indirectly calls `func1()`. Therefore, `func1` is marked with `declare target enter(func1) indirect(.true.)` so that it can be made available on the target device and called indirectly.

## Reproducer

```fortran
! This test checks the lowering of OpenMP Indirect Clause when used with the Declare Target directive

module functions
implicit none

interface
function func() result(i)
character(1) :: i
end function
end interface

contains
function func1() result(i)
!$omp declare target enter(func1) indirect(.true.)
character(1) :: i
i = 'a'
return
end function
end module

program main
use functions
implicit none
procedure (func), pointer :: ptr1=>func1
character(1) :: val1

!$omp target map(from: val1)
val1 = ptr1()
!$omp end target

end program
```

## Command

```bash
flang -fopenmp -fopenmp-version=52 omp-clause-indirect.f90
```

## Actual behavior

Flang emits a `not yet implemented` message and then aborts:

```text
warning: OpenMP support for version 52 in flang is still incomplete
error: ... ClauseProcessor.h:236: not yet implemented: Unhandled clause INDIRECT in DECLARE TARGET construct
LLVM ERROR: aborting
```

## Expected behavior

Flang should not abort.

Ideally, Flang should lower the `INDIRECT` clause on `DECLARE TARGET`

Start by running the supplied Fortran reproducer with `flang -fopenmp -fopenmp-version=52 omp-clause-indirect.f90` and inspect the lowering path for `DECLARE TARGET` and its `INDIRECT` clause. Done means the compiler no longer aborts on this input and, ideally, lowers the clause as expected.

Written by the indexing model from the issue text.