Failure building fortran-sdl2 binding interfaces

`flang` fails to build [fortran-sdl2](https://github.com/interkosmos/fortran-sdl2). For example, as STC

```
$ flang `sdl2-config --cflags` -O3 -fPIC -c src/c_util.f90
error: Semantic errors in src/c_util.f90
./src/c_util.f90:12:27: error: PARAMETER attribute was already specified on 'c_uint8_t'
integer, parameter :: c_uint8_t = c_int8_t
^^^^^^^^^
./src/c_util.f90:12:27: error: 'c_uint8_t' is use-associated from module 'iso_c_binding' and cannot be re-declared
integer, parameter :: c_uint8_t = c_int8_t
^^^^^^^^^
[...]

```

The same command works with `gfortran`, indeed I use it for my programs!

I tested this in MSYS2/UCRT64 with

```
$ flang --version
flang version 21.1.1
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: C:/msys64/ucrt64/bin
```

Reproduce the failure with flang 21.1.1 using the shown sdl2-config command, then inspect src/c_util.f90 around line 12 and the handling of iso_c_binding declarations. Compare the diagnostic with gfortran's behavior. Done means the fortran-sdl2 binding interfaces build successfully with flang without the redeclaration errors.

Written by the indexing model from the issue text.