LLVM backends: work around poorly designed C API

As part of the Opaque Pointers upgrade documentation, LLVM says that the
function LLVMGetGEPSourceElementType() can be used to obtain element
type information in lieu of LLVMGetElementType(), however, this function
actually returns the struct type, not the field type. The GEP
instruction does store the information we need, however, this is not
exposed in the C API. It seems like they accidentally exposed the wrong
field, because one would never need the struct type since one must
already pass it directly to the GEP instruction, so one will always have
it handy, whereas one will usually not have the field type handy.

src/zig_llvm.cpp

@@ -1195,6 +1195,10 @@ void ZigLLVMSetCallElemTypeAttr(LLVMValueRef Call, size_t arg_index, LLVMTypeRef
             Attribute::get(call_inst->getContext(), Attribute::ElementType, llvm_type));
 }
 
+LLVMTypeRef ZigLLVMGetGEPResultElementType(LLVMValueRef GEP) {
+    return wrap(unwrap<GEPOperator>(GEP)->getResultElementType());
+}
+
 void ZigLLVMFunctionSetPrefixData(LLVMValueRef function, LLVMValueRef data) {
     unwrap<Function>(function)->setPrefixData(unwrap<Constant>(data));
 }