WIP: Add `@code_llvm` macro

This allows you to do:
```julia
In  [74]: @code_llvm 1 + 1

define i64 @"julia_+672"(i64, i64) {
top:
  %2 = add i64 %1, %0, !dbg !2075
  ret i64 %2, !dbg !2075
}
```
Reference issue: #5674

I based this entirely after the implementation of `@which`, but it seems like it could be a bit cleaner. If we get some more thorough review here I can accordingly add the other code_ macros.