[Flang] Incorrect value of C_INT_FAST*_T

## Summary

The values of `C_INT_FAST16_T` and `C_INT_FAST32_T` are inconsistent with the companion processor (i.e., Clang). This issue may be related to #58282.

### Reproducer

* test.f90

```fortran
use iso_c_binding
integer(c_int_fast16_t) :: i1
integer(c_int_fast32_t) :: i2
print *,'c_int_fast16_t=',c_sizeof(i1)
print *,'c_int_fast32_t=',c_sizeof(i2)
end
```

* test.c

```c
#include
#include
int main(void) {
printf("sizeof(int_fast16_t)=%zu\n", sizeof(int_fast16_t));
printf("sizeof(int_fast32_t)=%zu\n", sizeof(int_fast32_t));
return 0;
}
```

* commands

```console
$ flang --version
flang version 23.0.0git (https://github.com/llvm/llvm-project.git ecdd6403fd213f90243a3d354d4db3483b89471f)
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /path/to/llvm/build/bin
Build config: +assertions
$ flang test.f90 && ./a.out
c_int_fast16_t= 2
c_int_fast32_t= 4
$ clang test.c && ./a.out
sizeof(int_fast16_t)=8
sizeof(int_fast32_t)=8
```

## 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
c_int_fast16_t= 8
c_int_fast32_t= 8
$ gcc test.c && ./a.out
sizeof(int_fast16_t)=8
sizeof(int_fast32_t)=8
```

* 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
c_int_fast16_t= 8
c_int_fast32_t= 8
$ icx -O0 test.c && ./a.out
sizeof(int_fast16_t)=8
sizeof(int_fast32_t)=8
```

Start with the test.f90 reproducer and compare its c_sizeof results with test.c compiled by clang on the shown AArch64 target. Trace Flang's iso_c_binding handling for C_INT_FAST16_T and C_INT_FAST32_T. Done means the Flang sizes match the companion C compiler, with regression coverage for the reported case.

Written by the indexing model from the issue text.