Enable passing function pointers as arguments.

Nobody has claimed this yet.

Hello,

The following code snippet :

````
int less () { .. }
int bsearch(int i,int (*compar)()) { .. }

int bug4(int i) {
int res = bsearch(1,less);
}
````

when running

````
Polygeist/build/bin/mlir-clang -S --raise-scf-to-affine -function=* -o=bug4.txt bug4.c
````

produces

````
clang version 15.0.0 (https://github.com/llvm/llvm-project.git 08ac66124874d70dab63c731da0244f9e29ef168)
...
loc("bug4.c":2:5): error: redefinition of symbol named 'less'
"builtin.module"() ({
"llvm.func"() ({
}) {function_type = !llvm.func, linkage = #llvm.linkage, sym_name = "less"} : () -> ()
"func.func"() ({
%0 = "arith.constant"() {value = 1 : i32} : () -> i32
"func.return"(%0) : (i32) -> ()
}) {function_type = () -> i32, llvm.linkage = #llvm.linkage, sym_name = "less"} : () -> ()
"func.func"() ({
^bb0(%arg0: i32, %arg1: !llvm.ptr>):
%0 = "arith.constant"() {value = 1 : i32} : () -> i32
%1 = "llvm.call"(%arg1) : (!llvm.ptr>) -> i32
"func.return"(%0) : (i32) -> ()
}) {function_type = (i32, !llvm.ptr>) -> i32, llvm.linkage = #llvm.linkage, sym_name = "bsearch"} : () -> ()
"func.func"() ({
^bb0(%arg0: i32):
%0 = "arith.constant"() {value = 1 : i32} : () -> i32
%1 = "llvm.mlir.addressof"() {global_name = @less} : () -> !llvm.ptr>
%2 = "func.call"(%0, %1) {callee = @bsearch} : (i32, !llvm.ptr>) -> i32
"func.return"(%2) : (i32) -> ()
}) {function_type = (i32) -> i32, llvm.linkage = #llvm.linkage, sym_name = "bug4"} : () -> ()
}) {dlti.dl_spec = #dlti.dl_spec<#dlti.dl_entry<"dlti.endianness", "little">, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>, #dlti.dl_entry : vector<2xi32>>>, llvm.data_layout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128", llvm.target_triple = "x86_64-unknown-linux-gnu"} : () -> ()
````

The C source are below

[bug4 .c.txt](https://github.com/wsmoses/Polygeist/files/8691926/bug4.c.txt)

Start by reproducing the command shown with the attached bug4.c source and inspect the emitted MLIR around the duplicate less symbols. Trace how Polygeist lowers the function definition and function-pointer argument. Done means the example compiles without a symbol redefinition and preserves the indirect call through the function pointer.

Written by the indexing model from the issue text.