Prepare struct_gep for opaque pointers

Imlement struct_gep using LLVMBuildStructGEP2 which takes an explicit
type argument instead of deriving it from a pointer type.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1408,8 +1408,9 @@ extern "C" {
         NumIndices: c_uint,
         Name: *const c_char,
     ) -> &'a Value;
-    pub fn LLVMBuildStructGEP(
+    pub fn LLVMBuildStructGEP2(
         B: &Builder<'a>,
+        Ty: &'a Type,
         Pointer: &'a Value,
         Idx: c_uint,
         Name: *const c_char,