Prepare inbounds_gep for opaque pointers

Implement inbounds_gep using LLVMBuildInBoundsGEP2 which takes an
explicit type argument instead of deriving it from a pointer type.

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -177,7 +177,12 @@ pub trait BuilderMethods<'a, 'tcx>:
     );
 
     fn gep(&mut self, ty: Self::Type, ptr: Self::Value, indices: &[Self::Value]) -> Self::Value;
-    fn inbounds_gep(&mut self, ptr: Self::Value, indices: &[Self::Value]) -> Self::Value;
+    fn inbounds_gep(
+        &mut self,
+        ty: Self::Type,
+        ptr: Self::Value,
+        indices: &[Self::Value],
+    ) -> Self::Value;
     fn struct_gep(&mut self, ty: Self::Type, ptr: Self::Value, idx: u64) -> Self::Value;
 
     fn trunc(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;