[Flang] Compilation error for generic interface blocks that specify private procedure pointers

## Summary

Compilation abnormally terminates when a generic interface block specifies a private procedure pointer and another generic interface block exists for the same generic procedure.

* 15.4.3.2 Interface block

> C1509 (R1501) An interface-specification in a generic interface block shall not specify a procedure that was specified previously in any accessible interface with the same generic identifier.

> An interface body in a generic or specific interface block specifies the EXTERNAL attribute and an explicit specific interface for an external procedure, dummy procedure, or procedure pointer.

### Reproducer

* test.f90

```fortran
MODULE mod
PROCEDURE(r_sub),PRIVATE,POINTER :: pprc
INTERFACE gnr
PROCEDURE pprc
END INTERFACE
CONTAINS
SUBROUTINE r_sub(d)
REAL :: d
d = 5.0
END SUBROUTINE

SUBROUTINE set_pprc()
pprc => r_sub
END SUBROUTINE

SUBROUTINE i_sub(d)
INTEGER :: d
d = 2
END SUBROUTINE
END MODULE

PROGRAM main
USE mod
IMPLICIT NONE
REAL :: a
INTERFACE gnr
PROCEDURE i_sub
END INTERFACE

CALL set_pprc()
CALL gnr(a)

IF(a /= 5.0) THEN
PRINT*,"ERROR"
END IF
PRINT*,"pass"
END PROGRAM
```

* commands

```console
$ flang --version
flang version 23.0.0git (https://github.com/llvm/llvm-project.git 349542f7e122b8fda1d60f0b37c8f92ad65635d6)
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /path/to/llvm/build/bin
Build config: +assertions
$ flang test.f90 && ./a.out
error: loc("/path/to/test.f90":2:39): redefinition of symbol named '_QMmodEpprc'
error: verification of lowering to FIR failed
```

## Non-problematic Conditions

The issue does not occur under the following conditions:
* Removing the PRIVATE attribute from the pointer `pprc`.
* Moving the interface block in the main program to the module.
* Calling the specific procedure instead of the generic procedure.

## Other Compilers

* GFortran
```console
$ gfortran --version
GNU Fortran (GCC) 15.2.0
Copyright (C) 2025 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gfortran test.f90 && ./a.out
pass
```

* Intel Fortran
```console
$ ifx --version
ifx (IFX) 2025.3.2 20260112
Copyright (C) 1985-2026 Intel Corporation. All rights reserved.

$ ifx -O0 test.f90 && ./a.out
pass
```

Start by compiling the supplied test.f90 reproducer with flang and confirm the duplicate-symbol and FIR verification errors. Trace the lowering path for the private procedure pointer used in the module generic interface, then add or run a regression test showing that the program compiles and prints "pass" without the redefinition error.

Written by the indexing model from the issue text.