LLVM will move to "opaque pointers" in the future

[Johan Engelen]

Hi all,
   FYI:  
https://github.com/llvm/llvm-project/blob/main/llvm/docs/OpaquePointers.rst

Instead of having to specify the type of the data pointed too 
"i8*", "ptr" just says it's a pointer. This prevents a bunch of 
bitcasts in the IR (which LDC also often has to do). But it also 
means we can no longer rely on an LLVM IR to contain type 
information, for example when we generate a GEP:

```cpp
llvm::GetElementPtrInst *DtoGEP(LLValue *ptr, 
llvm::ArrayRef<LLValue *> indices,
                                 const char *name, 
llvm::BasicBlock *bb) {
   LLPointerType *p = isaPointer(ptr);
   auto gep = llvm::GetElementPtrInst::Create(p->getElementType(), 
...
// the p->getElementType() will no longer work somewhere in the 
future
```


 From the LLVM document:

Frontend Migration Steps
If you have your own frontend, there are a couple of things to do 
after opaque pointer types fully work.

Don't rely on LLVM pointee types to keep track of frontend 
pointee types
Migrate away from LLVM IR instruction builders that rely on 
pointee types
For example, IRBuilder::CreateGEP() has multiple overloads; make 
sure to use one where the source element type is explicitly 
passed in, not inferred from the pointer operand pointee type


cheers,
   Johan