Add function signature to code_native and code_llvm (#50630)
When getting LLVM IR from `@code_llvm`, it's sometimes necessary to know
exactly which `f` is being compiled. This prints the type signature of a
function in front of `code_llvm` and `code_native` to assist in that
process.
<details>
<summary>Example</summary>
```julia
julia> @code_llvm zeros(64)
```
```llvm
; Function Signature: zeros(Int64)
; @ array.jl:629 within `zeros`
define nonnull {}* @julia_zeros_121(i64 signext %"dims[1]::Int64") #0 {
top:
; @ array.jl:629 within `zeros` @ array.jl:631 @ array.jl:634
; ┌ @ boot.jl:484 within `Array` @ boot.jl:475
%0 = call nonnull {}* inttoptr (i64 140604991500960 to {}* ({}*, i64)*)({}* inttoptr (i64 140604630439728 to {}*), i64 %"dims[1]::Int64")
; └
; @ array.jl:629 within `zeros` @ array.jl:631 @ array.jl:635
; ┌ @ array.jl:392 within `fill!`
; │┌ @ abstractarray.jl:318 within `eachindex`
; ││┌ @ abstractarray.jl:134 within `axes1`
; │││┌ @ abstractarray.jl:98 within `axes`
; ││││┌ @ array.jl:191 within `size`
%1 = bitcast {}* %0 to { i8*, i64, i16, i16, i32 }*
%.length_ptr = getelementptr inbounds { i8*, i64, i16, i16, i32 }, { i8*, i64, i16, i16, i32 }* %1, i64 0, i32 1
%.length = load i64, i64* %.length_ptr, align 8
; │└└└└
; │┌ @ range.jl:897 within `iterate`
; ││┌ @ range.jl:674 within `isempty`
; │││┌ @ operators.jl:378 within `>`
; ││││┌ @ int.jl:83 within `<`
%.not.not = icmp eq i64 %.length, 0
; │└└└└
br i1 %.not.not, label %L30, label %L13.preheader
L13.preheader: ; preds = %top
%2 = bitcast {}* %0 to i8**
%.data1013 = load i8*, i8** %2, align 8
; │ @ array.jl:394 within `fill!`
%3 = shl nuw i64 %.length, 3
; │ @ array.jl:393 within `fill!`
; │┌ @ array.jl:1019 within `setindex!`
call void @llvm.memset.p0i8.i64(i8* nonnull align 8 %.data1013, i8 0, i64 %3, i1 false)
; └└
; @ array.jl:629 within `zeros`
br label %L30
L30: ; preds = %L13.preheader, %top
ret {}* %0
}
```
</details>
