[CIR] CIR drops pointee const value qualifier

Take this program:

```c
int strcmp(const char *, const char *);
int (*p)(const char *, const char *) = strcmp;
```

Running the following:

```sh
~/llvm-project/build-cir/bin/clang -std=gnu11 -fclangir -emit-cir test.c -o -
```

I got this:

```mlir
!s32i = !cir.int
!s8i = !cir.int
...
cir.global external dso_local @p = #cir.global_view<@strcmp> : !cir.ptr, !cir.ptr) -> !s32i>> {alignment = 8 : i64} loc(#loc5)
cir.func private @strcmp(!cir.ptr {llvm.noundef}, !cir.ptr {llvm.noundef}) -> !s32i attributes {"cir.target-cpu" = "x86-64", "cir.target-features" = "+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87", "cir.tune-cpu" = "generic", nothrow} loc(#loc6)
} loc(#loc)
#loc = loc("/home/takashi/scratch/test.c":0:0)
#loc1 = loc("test.c":3:1)
#loc2 = loc("test.c":4:63)
#loc3 = loc("test.c":1:1)
#loc4 = loc("test.c":1:38)
#loc5 = loc(fused[#loc1, #loc2])
#loc6 = loc(fused[#loc3, #loc4])
```

Taking a look at the clang AST, all of these are properly preserved:

Grepping through the ast dump:

```sh
clang -std=gnu11 -Xclang -ast-dump -fsyntax-only test.c
```

The `const char *` is properly preserved, so I assume something is happening in between that's removing the const qualifier for the CIR side.

```
26:`-VarDecl 0x559e700dce58 line:3:7 p 'int (*)(const char *, const char *)' cinit
27: `-CStyleCastExpr 0x559e700dd088 'int (*)(const char *, const char *)'
28: `-ImplicitCastExpr 0x559e700dd070 'int (*)(const char *, const char *)' part_of_explicit_cast
29: `-DeclRefExpr 0x559e700dd008 'int (const char *, const char *)' Function 0x559e700dcb38 'strcmp' 'int (const char *, const char *)'
```

Start with the test.c reproducer and run the provided clang command, then compare its CIR output with the AST dump from the second command. Trace the CIR lowering between the preserved C function types and the emitted cir.func and cir.global types; done means both retain const on the pointee types.

Written by the indexing model from the issue text.